Use MESA_PBUFFER_ALLOC/FREE macros to allocate all framebuffer and texture

Use MESA_PBUFFER_ALLOC/FREE macros to allocate all framebuffer and texture
memory.  These can be overridden by applications which need to manage this
memory specially.  Contributed by Gerk Huisma.
Also, new code for 8-bit -> 16-bit/channel texture image storage which
fills in the least-significant bits properly.