Edit Mode: Rework display to differentiate selection modes

This removes the large edges and instead use colors to hint in which
selection mode the user is.

The component in each individual selection mode is more prominent to add
more emphasis. The other components are less prominent and dimmed.

A minor default theme changes needed to be made to keep clarity in all
combinations.

Forcing old behavior (no selection mode hit) could be done quite easily
by just making the 2 booleans true (selectFaces and selectEdges).

Reviewers: campbellbarton, billreynish

Reviewed By: campbellbarton, billreynish

Subscribers: ThinkingPolygons

Maniphest Tasks: T1234

Differential Revision: https://developer.blender.org/D4526

GPUBuffers: Remove wrong assert

The assert was not true if the pbvh node had no triangle.

Also update the comment to reflect that.

Fix T63284: Eevee: Crash when subsurface pass is enabled

Was caused by a NULL texture used as uniform.

Fix T63377: "Principled Volume" node not working anymore in EEVEE

I'm not sur how it ever worked before. There was no texture bound to these
sampdensity and sampflame when no volume simulation was happening.

This fixes the issue using 1x1x1 dummy textures.

DRW: Opti: Fix hotspot in DRW_mesh_batch_cache_get_surface_shaded

The hotspot was generated by mesh_cd_layers_type_overlap who was testing
way more data than it should have.

Here we reduce the whole CD layer mask to a 32bit bitflag that is easily
or'ed and tested.

Bonus point: We use atomic operation to merge the mask in order to allow
future multi-threading. (this was a TODO)

In the scene attached to T58188 this removes 5% of CPU time.

DRW: Opti: Replace bound tex/ubo tracking array by bitfields

release_texture_slots() and release_ubo_slots() were one hotspot when
drawing taking ~9% of total CPU counters for no reason.

This was because of the loops using GPU_max_textures that was overkill and
slow.

Replace those by a simple 64bit bitwise OR operation.

GPU: Fix typo

Fix T61035 Draw manager crash opening file with curves

Move free callback call to GPU_batch_discard to prevent the crash.

The issue was that clearing can happen after referencing to an instance
buffer and that's perfectly legal.

Fix T62892 EEVEE HDRI lightning glitch

Clamp the texture at sampling time. This is not the best way to do it but
this is the fastest/simplest. The cost is rather negligeable.

Fix T63108 Eevee: Wrong Reflection plane matrix if camera view is not centered

Invert X axis of the Window Matrix (AKA Projection matrix) instead of the
View matrix.