mesa: generate error if pbo offset is not aligned with the size of specified type

mesa: generate error if pbo offset is not aligned with the size of specified type

v2: quote the spec; explicitly exclude the GL_BITMAP case to make code
    more readable. (comments from Ian)

v3: Cast the offset by GLintptr to remove the compile warning(comments
    from Brian).

    I also found that I should use _mesa_sizeof_packed_type() instead,
    as it includes packed pixel type, like GL_UNSIGNED_SHORT_5_6_5.

Signed-off-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>
Reviewed-by: Brian Paul <brianp@vmware.com>