Replace MESA_FORMAT_DEPTH_COMPONENT_FLOAT32 with 32-bit integer format.

Replace MESA_FORMAT_DEPTH_COMPONENT_FLOAT32 with 32-bit integer format.
This allows render to depth texture (we don't support floating pt. Z buffers).
Rename MESA_FORMAT_DEPTH_COMPONENT16/32 as MESA_FORMAT_Z16/32.
Software fallback for glCopyTexImage now uses integer temporary image instead
of float, eliminates a lot of float/int conversions.