Fix Cycles X crash viewing render result prior rendering

Cycles X: Real memory saving when using tiled rendering

The general idea is to delay reading EXR files for until after all
view layers are rendered. Once they are all rendered, BlenderSession
frees up as much memory as possible, and initiates processing of the
on-disk files.

The processing includes reading the file, optional denoising of the
full frame (if the denoising is enabled), and writing the result via
the write callback.

The processing is done as a state-machine which routes specific calls
to a full-frame processing, which allows to re-use same tile write
callback in the software integration.

In order to be able to know which view layer and render view is being
written the API has been extended to write layer and view names. Done
via BufferParams, as layer and view concepts are quite typical for the
EXR files.

The BufferParams also contains all fields needed for buffers access
outside of the scene graph.

Differential Revision: https://developer.blender.org/D12503

Fix wrong display when rendering multiple view layers in Cycles X

The issue was that the texture used by GPUDisplay was never cleared and
was only used for partial updates.

The actual reasoning of that is a bit more tricky than it looks on a
first glance since the GPUDisplay was re-created together with session
via session_reset so one would think it is weird to have a texture from
previous view layer to appear in the new GPUDisplay. Probably caused by
the fact that OpenGL is the same for all view layers, and texture ID is
happened to be the same.

Anyhow, the way texture is created is considered to have an undefined
behavior by the standard when is attempted to be used without explicit
data assignment.

This change makes it so new texture is always guaranteed to be zeroed
and that the GPU display is cleared when rendering multiple views from
within the same session.

The clearing is done by a GPUDisplay subclass, as the parent does not
know details of how to perform the zeroing. For the CPU the zeroing is
done on a mapped memory, as it makes it easy and does not require any
extra OpenGL requirements.

For the graphics interop things a bit more difficult as attempts to map
texture will make graphics interop to fail out (it really wants to be
an exclusive thing which takes care of the memory mapping). So for the
graphics interop clearing is done when memory is mapped by the interop
implementation.

Differential Revision: https://developer.blender.org/D12514

Fix persistent data not allowing to change sample count and time limit

The root cause was that `session->params` were never updated to the new
parameters.

Made it so `session->reset()` will make sure the session is fully up to
date with the desired parameters.

Investigation by @dingto, code by self.

Differential Revision: https://developer.blender.org/D12515

Fix pass accessor with 0 samples

Avoid division by zero when calculating scale, and avoid multiplication
by inf later on.

The pixel processing functions are changed so that they return fully
transparent pixel for pixels where number of samples is 0. This allows
to have matched behavior of non-finished tiles with master, without
relying on non-finite math.

The extra condition when calculating scale is inevitable, and hopefully
the pixel processing function's if statement gets folded into it as
well ()due to force-inline).

Differential Revision: https://developer.blender.org/D12518

Fix race condition between draw and render threads

Caused crashes like the one William was fixing in the D12498, or the
crash when rendering multiple view layers.

Need to guarantee that the render engine is not freed during drawing,
and that the external engine is not in the middle of re-allocation due
to `update()` which might cause session reset (happens in Cycles).

Had to introduce a separate lock. The reason for this comes from the
fact that we need to acquire the engine early on in the draw manager,
and release it only when drawing is done. However, some other engines
(like overlay) might be requesting ImBuf for the image space, so that
they know dimensions. Such acquisition is guarded by the resultmutex.
This means reusing `resultmutex` for the `RenderEngine::draw()` would
cause a recursive lock.

Not entirely happy with implicit release in the external engine code,
but not sure there is an existing way of making it explicit without
introducing new draw engine callback.

Differential Revision: https://developer.blender.org/D12521

Fix invalid address error in Cycles X GPUDisplay update

Make sure no display update happens after full buffer was written.
Makes it possible to free memory used by path trace works without
causing issues.

Cleanup: Naming

Fix missing mutex lock for viewport OpenGL context

A mistake in D12468: the viewport uses GHOST to operate on contexts,
bypassing draw manager state,

Differential Revision: https://developer.blender.org/D12505

Cleanup: Typo fix in a comment