add notes about cavity shader

include/shader_program.h

@@ -6,6 +6,12 @@
 #include "types.h"
 
 
+// TODO: implement a 'cavity' effect for the default shader
+// 	https://blender.community/c/rightclickselect/J9bbbc/
+// 	https://developer.blender.org/rBf1fd5ed74fb0afd602f53860d0b2db46189c218a
+// 	https://developer.blender.org/diffusion/B/browse/master/source/blender/draw/engines/workbench/shaders/workbench_cavity_lib.glsl
+// 	https://www.casual-effects.com/research/McGuire2011AlchemyAO/VV11AlchemyAO.pdf
+// 	https://github.com/evanw/madebyevan.com/blob/master/src/templates/shaders-curvature.jade
 struct default_shader_program
 {
 	GLuint program_id;