mesa: Unbind deleted textures from the shader image units.

mesa: Unbind deleted textures from the shader image units.

From ARB_shader_image_load_store:

   If a texture object bound to one or more image units is deleted by
   DeleteTextures, it is detached from each such image unit, as though
   BindImageTexture were called with <unit> identifying the image unit
   and <texture> set to zero.

Reviewed-by: Chris Forbes <chrisf@ijw.co.nz>
Reviewed-by: Paul Berry <stereotype441@gmail.com>