Wrapped related global vars into struct.

Move index from DrawEngineType to DRWRegisteredDrawEngine.

Fix incorrect merge: missing function `DRW_view_get_active`.

DRW: Fix implicit convertion warning on MSVC

Merge branch 'master' into draw-viewport-data

Merge branch 'master' into draw-viewport-data

# Conflicts:
# source/blender/draw/DRW_engine.h
# source/blender/draw/intern/draw_manager.c
# source/blender/draw/intern/draw_manager.h

Cleanup: Remove compiler warning and fix some comments

DRW: Move buffer & temp textures & framebuffer management to DrawManager

This is a necessary step for EEVEE's new arch. This moves more data
to the draw manager. This makes it easier to have the render or draw
engines manage their own data.

This makes more sense and cleans-up what the GPUViewport holds

Also rewrites the Texture pool manager to be in C++.

This also move the DefaultFramebuffer/TextureList and the engine related
data to a new `DRWViewData` struct. This struct manages the per view
(as in stereo view) engine data.

There is a bit of cleanup in the way the draw manager is setup.
We now use a temporary DRWData instead of creating a dummy viewport.

Differential Revision: https://developer.blender.org/D11966

DRW: Fix memory leak of GPUTextures

The textures needs to be released by iterating. Not by using the
free callback.

DRW: Move mempool datas to a DRW managed struct.

Same idea as previous commit. This cleans-up the interface and put all
viewport related data inside the `DRWData` struct.

The draw manager is responsible for freeing it. That is the main point
of this all. In the future, we can have custom freeing method for each
engine.

This also move the DefaultFramebuffer/TextureList and the engine related
data to a new `DRWViewData` struct. This struct manages the per view
(as in stereo view) engine data.

There is a bit of cleanup in the way the draw manager is setup.
We now use a temporary DRWData instead of creating a dummy viewport.

DRW: Move GPUViewport texture pool to DRW module

This makes more sense and cleans-up what the GPUViewport holds.

Also rewrite it to be in C++.