mesa: fix image unpacking when storing compressed textures

mesa: fix image unpacking when storing compressed textures

This fixes failures found with the new piglit texsubimage test.

Two things were broken:
1. The dxt code doesn't handle sources images where width != row stride.
   Check for that and take the _mesa_make_temp_ubyte_image() path to get
   an image where width = rowstride.
2. If we don't take the _mesa_make_temp_ubyte_image() path we need to
   take the source image unpacking parameters into account in order to
   get the proper starting memory address of the source texels.

Note: This is a candidate for the 7.11 branch.