color: add from sRGB to blending color transformation

This is needed when the compositor produces any content internally:
- the lines in triangle fan debug
- the censoring color fill (unmet HDCP requirements)

Solid color surfaces do not need this special-casing because
weston_surface is supposed to carry color space information, which will
get used in gl_shader_config_init_for_view().

This makes sure the internally produced graphics fit in, e.g on a
monitor in HDR mode.

For now, just ensure there is an identity transformation. Actual
implementations in GL-renderer will follow later.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.com>

libweston/renderer-gl/gl-renderer.c

@@ -1034,6 +1034,9 @@ draw_paint_node(struct weston_paint_node *pnode,
 	else
 		filter = GL_NEAREST;
 
+	/* for triangle_fan_debug(), maybe_censor_override() */
+	assert(pnode->output->from_sRGB_to_blend == NULL);
+
 	if (!gl_shader_config_init_for_paint_node(&sconf, pnode, filter))
 		goto out;