gl-renderer: add note about fallback shader color

Now that pieces of color management implementation start to land, the
fallback shader becomes even more special than before. It is the only
case where the compositor ignores color management.

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

libweston/renderer-gl/gl-shaders.c

@@ -463,6 +463,15 @@ gl_renderer_use_program(struct gl_renderer *gr,
 	if (!shader) {
 		weston_log("Error: failed to generate shader program.\n");
 		gr->current_shader = NULL;
+
+		/*
+		 * We only have one fallback shader, so it cannot do correct
+		 * color on color managed outputs. Hence, what is painted
+		 * with this one will have undefined look. Therefore the
+		 * fallback is important to not be too bright as that might
+		 * be shocking on a monitor in HDR mode.
+		 */
+
 		shader = gr->fallback_shader;
 		glUseProgram(shader->program);
 		glUniform4fv(shader->color_uniform, 1, fallback_shader_color);