Blender Git Loki

Avoid own global variable, use existing DRWManager struct

Only cache framebuffers for layers that aren't always redrawn

Get draw-layers to work with multiple viewports

Use hash, not list to store and lookup DRWLayer items

Fix memory leak when only drawing gizmos

Merge branch 'master' into temp-gizmo-decoupled-redraws

Draw-layers: Get it all to work!

Create the framebuffers for caching viewport rendering results, and
"composite" all layers together once each is drawn. Compositing is a bit
of a big word for this, since we basically just draw all layers on top
of each other with alpha blending.
After talking with Jeroen, we decided to keep things simple for now, we
can add more features (like per-layer color-management) and do small
improvements later.

Merge branch 'master' into temp-gizmo-decoupled-redraws

Initial introduction of DRWLayer abstraction

Used to wrap different drawing steps, i.e. the main scene drawing,
editor overlays and debug text. Later overlays should get their own
layer.
Right now only the drawing commands are wrapped into per layer
callbacks. The idea is to let each draw into different framebuffers and
to composit them together at the end, so that we can avoid redrawing all
layers if only one needs updating.

Add separate region redraw tag for UI overlays (gizmos)