i965: asst. fixes, work-arounds for FBOs and render to texture
OpenGL allows mixing and matching depth and stencil renderbuffers in framebuffer objects while the hardware really only supports interleaved depth/stencil buffers. This makes for some tricky buffer management. An extra wrinkle is the situation where the user allocates a 16bpp depth texture or renderbuffer then tries to render to it along with a stencil buffer. We'd have to promote the 16bpp Z values to 24-bit Z values and mix in the stencil values to setup the depth/stencil renderbuffer. There's no support for that now, so always allocate 32bpp depth textures/ renderbuffers for now.
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