st/mesa: provide GL_OES_copy_image support by caching the original ETC data

The additional provision of GL_OES_copy_image is that it work for ETC.
However many desktop GPUs don't have native ETC support, so st/mesa does
the decoding by hand. Instead of discarding the compressed data, keep it
around in CPU memory. Use it when performing image copies.

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Acked-by: Marek Olšák <marek.olsak@amd.com>

docs/GL3.txt

@@ -257,7 +257,7 @@ GLES3.2, GLSL ES 3.2:
   GL_KHR_debug                                          DONE (all drivers)
   GL_KHR_robustness                                     DONE (i965)
   GL_KHR_texture_compression_astc_ldr                   DONE (i965/gen9+)
-  GL_OES_copy_image                                     DONE (i965)
+  GL_OES_copy_image                                     DONE (all drivers)
   GL_OES_draw_buffers_indexed                           DONE (all drivers that support GL_ARB_draw_buffers_blend)
   GL_OES_draw_elements_base_vertex                      DONE (all drivers)
   GL_OES_geometry_shader                                started (idr)

docs/relnotes/12.1.0.html

@@ -52,6 +52,7 @@ Note: some of the new features are only available with certain drivers.
 <li>GL_ARB_ES3_1_compatibility on i965</li>
 <li>GL_EXT_window_rectangles on nv50, nvc0</li>
 <li>GL_KHR_texture_compression_astc_sliced_3d on i965</li>
+<li>GL_OES_copy_image on nv50, nvc0, r600, radeonsi, softpipe, llvmpipe</li>
 </ul>
 
 <h2>Bug fixes</h2>