Edit Mode: Make edit edges black by default

This is in order to have better contrast now that
the edges are half transparent if not
in edge selection.

DRW: Expose VBO garbage collection timings

This adds user side options to tweak the behavior
of the vbo garbage collection.

DRW: Make shaded batch validation more correct

This is in order to support garbage collection of unused Custom data layer uploaded to the GPU.

Actual Garbage Collection is not added by this commit.

DRW: Add batch garbage collection

This is only working for shading batches for the moment and only if some Custom data layer are not needed anymore.

The collection rate is hardcoded at 60 sec but could be exposed to the user.

This system can be extended and discard most unused batches in the future.

This commit is in prevision of removing BKE_MESH_BATCH_DIRTY_SHADING when changing shader parameters.

Edit Mode: Change face mode edges display

This improve visibility in edit face select mode by using the face select color
instead of the edge select color (which is in default theme a bit more red). Also
makes the selected edges in this mode a bit more opaque (0.75 instead of 0.4).
Full opacity is still reserved for edge select mode.

Edit Mode: Reduce Selected face alpha

This is to be less intrusive when doing UV mapping. They remain enough visible
in every select mode

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.