main/texstore: Split texture storage into three functions

main/texstore: Split texture storage into three functions

This commit splits the texture storage into three functions:
texstore_depth_stencil, texstore_compressed, and texstore_rgba.  Right now
this split seems artificial since we just have one function pointer per
format and there is no difference between these three categories.  However,
this split makes it much easier to write a more general function upload
path for one of these categories than the current function pointers.

Signed-off-by: Jason Ekstrand <jason.ekstrand@intel.com>
Reviewed-by: Brian Paul <brianp@vmware.com>