gl: Force U8 textures for U8_SRGB depths

The GL renderer is using FLOAT32 instead of GL_SRGB, which is screwing
up the node-editor by making it turn on high bit depth unconditionally.

So until someone fixes the GL renderer properly, do this quickfix.

gsk/gl/gskglrenderer.c

@@ -455,7 +455,10 @@ gsk_gl_renderer_render_texture (GskRenderer           *renderer,
       return texture;
     }
 
+  /* Don't use float textures for SRGB or node-editor turns on high 
+   * depth unconditionally. */
   if (gsk_render_node_get_preferred_depth (root) != GDK_MEMORY_U8 &&
+      gsk_render_node_get_preferred_depth (root) != GDK_MEMORY_U8_SRGB &&
       gdk_gl_context_check_version (self->context, "3.0", "3.0"))
     {
       gdk_format = GDK_MEMORY_R32G32B32A32_FLOAT_PREMULTIPLIED;