For Blender 2.8 we had to be compatible with very old OpenGL versions, and
triple buffer was designed to work without offscreen rendering, by copying
the the backbuffer to a texture right before swapping. This way we could
avoid redrawing unchanged regions by copying them from this texture on the
next redraws. Triple buffer used to suffer from poor performance and driver
bugs on specific cards, so alternative draw methods remained available.
Now that we require newer OpenGL, we can have just a single draw method
that draw each region into an offscreen buffer, and then draws those to
the screen. This has some advantages:
* Poor 3D view performance when using Region Overlap should be solved now,
since we can also cache overlapping regions in offscreen buffers.
* Page flip, anaglyph and interlace stereo drawing can be a little faster
by avoiding a copy to an intermediate texture.
* The new 3D view drawing already writes to an offscreen buffer, which we
can draw from directly instead of duplicating it to another buffer.
* Eventually we will be able to remove depth and stencil buffers from the
window and save memory, though at the moment there are still some tools
using it so it's not possible yet.
* This also fixes a bug with Eevee sampling not progressing with stereo
drawing in the 3D viewport.
Differential Revision: https://developer.blender.org/D3061
