mesa: replace Driver.GetTexImage with GetTexSubImage()

mesa: replace Driver.GetTexImage with GetTexSubImage()

The new driver hook has x/y/zoffset and width/height/depth parameters
for the new glGetTextureSubImage() function.

The meta code and gallium state tracker are updated to handle the
new parameters.

Callers to Driver.GetTexSubImage() pass in offsets=0 and sizes equal
to the whole texture size.

v2: update i965 driver code, s/GLint/GLsizei/ in GetTexSubImage hook

Reviewed-by: Ilia Mirkin <imirkin@alum.mit.edu>