View3D Grid: fix imperial grid drawing

This is a step to finish the D4325 and fix the T61286.
Currently the grid is highlighted in steps of 10 out of 10, which is wrong for the imperial units as seen in `buImperialLenDef`.
The idea of the code is to pass the steps of the grid already dimensioned as a uniform.

Another important thing to note is that subdivisions now only affect the grid without unity.
This matches the 2.7x Blender versions.

No performance loss (almost no gain too).

Reviewers: fclem

Subscribers: zlsa, rl.amorato

Differential Revision: https://developer.blender.org/D4379

Fix T68782: false occlusion in selecting vertices and edges with objects in any wire view

Fix check that validates a selection index

Fix T68684: Vertex snapping with median not working if the object is not at center of the world

DRW: New function DRW_culling_min_max_test

For testing intersection with frustrum planes without having to transform all bound box vertices into global space.

Edit Mesh Selection: Refactor: Redraw idmap buffer at runtime with only objects inside the rect

But in the future the selection code may also be used in object mode (eg for snapping).
So to avoid using too much VRAM resources, it is good to avoid drawing all objects in the viewport.

The solution was to create an array with only objects that are detected within the selection area.
If the selection operator is modal, objects already detected are not removed from the array until view3d is moved or orbited.
To detect the object, its BoundBox is tested.
Since the Select Engine does not have a dedicated depth texture, whenever a new object is "found" the depth of the objects in the array already drawn is redrawn.

Reviewers: campbellbarton, fclem

Reviewed By: fclem

Differential Revision: https://developer.blender.org/D5435

Cleanup: Remove redundant headers

Cleanup: Silence C4115 warning

`'ParticleSystem': named type definition in parentheses`
And prevent the need for struct `Object` to be defined.

Fix check that validates a selection index

Cleanup: Remove redundant headers