Blender Git Loki

GPUImmediate: Use 2 Buffers For (Un)Strict

We used to have a single buffer that was shared between strict and
unstrict draw calls. This leads to many recreation events for the draw
buffers. This patch separates the Unstrict draw buffer from the strict
draw buffer.

This improves performance on Windows Intel 10th gen platform.
On a reference platfor before the patch I got 10 FPS, after this patch
it became 34fps. Note that the same test normally on a low end GPU can
get to 60fps so this does not solve all teh bottlenecks yet.

Reviewed By: Cl�ment Foucault

Differential Revision: https://developer.blender.org/D7421