tex-driver-gles: Fix subregion uploads without GL_EXT_unpack_subimage

When the GL_EXT_unpack_subimage extension is not available and a
subregion of a texture is uploaded then it should first copy the
subregion to a newly allocated bitmap. However it was then later still
trying to prepare the upload using the original src_x and src_y values
which would cause an assertion failure. This patch fixes it to just
reset those to zero if the subregion is first copied.

Reviewed-by: Robert Bragg <robert@linux.intel.com>

(cherry picked from commit 6f9a62db6f846f1d76e3ca16d9d8cdadf82a7009)

cogl/driver/gles/cogl-texture-driver-gles.c

@@ -206,6 +206,8 @@ _cogl_texture_driver_upload_subregion_to_gl (CoglContext *ctx,
                                    src_x, src_y,
                                    0, 0, /* dst_x/y */
                                    width, height);
+
+      src_x = src_y = 0;
     }
   else
     slice_bmp = prepare_bitmap_alignment_for_upload (ctx, source_bmp);