mesa/swrast: Move memory allocation outside the blit loop

mesa/swrast: Move memory allocation outside the blit loop

Assume the maximum pixel size (16 bytes per pixel).  In addition to
moving redundant malloc and free calls outside the loop, this fixes a
potential resource leak when a surface is mapped and the malloc fails.
This also makes blit_nearest look a bit more like blit_linear.

v2: Use MAX_PIXEL_BYTES instead of 16.  Suggested by Ken.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Brian Paul <brianp@vmware.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>