Fix paint cursor crash

The new paint cursor (introduced in rBe0c792135adf) could crash for 2d
painting without an active object.

Note there are still drawing asserts (because we are mixing 2d and 3d
drawing in 'paint_draw_cursor'), but these will be handled in a seperate
commit.

part of T69957

Reviewers: jbakker

Maniphest Tasks: T69957

Differential Revision: https://developer.blender.org/D5820

Fix T69687: Edit Curve culling

Curve edit points could disappear when the deformed curve is out of view
area. Fix similar to rB0f983e854052.

Reviewers: jbakker

Maniphest Tasks: T69687

Differential Revision: https://developer.blender.org/D5748

Fix eevee volumetrics distribution at 1.0

reported in T69662

Reviewers: fclem

Differential Revision: https://developer.blender.org/D5809

Fix (unreported) 'smooth vertex colors' operator not respecting vertex
paint mask selection

followup to rBr27bbe7cbd9b, might as well make this consistent across
all the color operations [with the exception of 'Dirty Vertex Colors'
which is python]

Reviewers: brecht

Differential Revision: https://developer.blender.org/D5786

Fix Particles: Keyed Physics crash when clicking on a particle slot
without a valid target

rB95b9680597f5 introduced code that would skip creation of GPUVertBuf
for ParticlePointCache if the keyed physics would not have a valid
target. Missing vertex buffer would lead to assert/crash.

This code is now removed (dont see a reason why this was done? afaict
2.79 also just displayed the particles without physics in this case and
this seems to be working just fine in 2.8 as well)

part of T69741

Reviewers: fclem

Maniphest Tasks: T69741

Differential Revision: https://developer.blender.org/D5781

Fix vertex paint: color transform operators not respecting vertex paint
mask selection

for 'Invert', 'Levels', 'Hue saturation Value' and 'Bright/Contrast',
face mask was respected, but vertex mask wasnt...

Same code as done in 'Set Vertex Colors'.

reported in T69835

Reviewers: brecht

Maniphest Tasks: T69835

Differential Revision: https://developer.blender.org/D5783

Fix Particles Keyed Physics target UI greyed out

Picking the 'Target Object' as well as the 'System' number was greyed
out as long as 'Use Timing' was disabled. 'Use Timing' is unrelated for
the above two, these should always be active...

part of T69741

Reviewers: sergey

Maniphest Tasks: T69741

Differential Revision: https://developer.blender.org/D5782

Fix T69752: Texture paint sampling colors always 'merged down'

Rgression from rBaf4dcc6073fa.

paint_sample_color > imapaint_pick_face uses the the selection buffer
(DRW_select_buffer_sample_point) and to get flat colors [select_id_flat] we
need to be in SCE_SELECT_FACE mode. This was already fine if you had
'Face Selection Masking' turned on, but got colors including lighting
when turned of [select_id_uniform].

There was already an exception in 'select_cache_init' that turns on
SCE_SELECT_FACE for weightpaint, we just need this for texture paint
(vertex paint) as well... Also moved the logic into
select_id_get_object_select_mode.

Note we were also asserting here:
BLI_assert failed: /blender/source/blender/draw/engines/select/
select_engine.c:174, select_cache_init(), at 'e_data.context.select_mode
!= 0'

Note also this is not working correctly for vertexpaint (yet), but has
been discussed in T69752 and there is a solution by @mano-wii in P1032.

Reviewers: mano-wii

Subscribers: mano-wii

Maniphest Tasks: T69752

Differential Revision: https://developer.blender.org/D5775

Fix T69582: Empties no longer support Rigid Body Constraints

This is caused by rB1342d1879e12 and would also break the whole
"Connect" workflow [which relies on empties]

Reviewers: mont29, brecht

Maniphest Tasks: T69582

Differential Revision: https://developer.blender.org/D5772

Fix assert hiding selected particles

was trying to get a boolean with RNA_enum_get (instead of
RNA_boolean_get)

Spotted while looking into T69680, T69432