Blender Git Loki

OpenGL: draw grid floor & axes with new immediate mode

Significant rewrite with some improvements.

Maintain visual hierarchy of the grid:
- emphasized lines draw atop normal lines
- axes draw atop all lines (same as before)

Draw axes only once, not twice.

Return early if nothing to draw.

Single draw call for the default case (grid floor with X and Y axes).

Z axis needs a second draw call because it uses 3D coordinates.

Part of T49043