Missing sokol samples for feature coverage

[floooh]

• [x] depth-only pass
• [x] sample from depth texture (can be combined with depth-only rendering) (FIXME: this requires further fixes in the backends)
• [ ] render to mip map
• [ ] render to array texture slice (with and without MSAA)
• [ ] render to 3d texture slice
• [ ] render without vertex buffer (no vertex pulling from buffers though, since GLES3 has no SSBOs)
• [ ] ???