cogl: Read pixels as per the stored format

By the looks of it, commit 95e9fa10ef was taping over an Intel DRI bug
that would make it return post-swizzling pixel data on glReadPixels().
There's been reports over time of that commit resulting in wrong colors
on other drivers, and lately Mesa >17.3 started showing the same symptoms
on Intel.

But texture swizzling works by changing parameters before fragment shaders
and reading pixels from an already drawn FBO/texture doesn't involve those.
This should thus use pixel_format_to_gl_with_target(), which will result in
correctly requesting the same pixel format than the underlying texture,
while still considering it BGRA for the upper layers in the swizzling case.

https://gitlab.gnome.org/GNOME/mutter/issues/72

Closes: #72

cogl/cogl/driver/gl/cogl-framebuffer-gl.c

@@ -1412,22 +1412,12 @@ _cogl_framebuffer_gl_read_pixels_into_bitmap (CoglFramebuffer *framebuffer,
   if (!cogl_is_offscreen (framebuffer))
     y = framebuffer_height - y - height;
 
-  required_format = ctx->driver_vtable->pixel_format_to_gl (ctx,
-                                                            format,
-                                                            &gl_intformat,
-                                                            &gl_format,
-                                                            &gl_type);
-#if HAVE_COGL_GL
-  /* As we are reading pixels, we want to consider the bitmap according to
-   * its real pixel format, not the swizzled channels we pretend face to the
-   * pipeline.
-   */
-  if ((ctx->driver == COGL_DRIVER_GL || ctx->driver == COGL_DRIVER_GL3) &&
-      (format == COGL_PIXEL_FORMAT_BGRA_8888 ||
-       format == COGL_PIXEL_FORMAT_BGRA_8888_PRE) &&
-      _cogl_has_private_feature (ctx, COGL_PRIVATE_FEATURE_TEXTURE_SWIZZLE))
-    gl_format = GL_BGRA;
-#endif
+  required_format = ctx->driver_vtable->pixel_format_to_gl_with_target (ctx,
+                                                                        framebuffer->internal_format,
+                                                                        format,
+                                                                        &gl_intformat,
+                                                                        &gl_format,
+                                                                        &gl_type);
 
   /* NB: All offscreen rendering is done upside down so there is no need
    * to flip in this case... */