Such that it actually works in apps which use both.

A separate buffer is allocated for stencil. The only exception is
the window-system-provided depth-stencil buffer, where depth and stencil
share the same buffer.

This fixes:
- fbo-depthstencil-GL_DEPTH24_STENCIL8-clear
- fbo-depthstencil-GL_DEPTH24_STENCIL8-drawpixels-FLOAT-and-USHORT
- fbo-depthstencil-GL_DEPTH24_STENCIL8-readpixels-24_8
- fbo-depthstencil-GL_DEPTH24_STENCIL8-readpixels-FLOAT-and-USHORT