add builtin GPU_SHADER_KEYFRAME_DIAMOND

Now we can draw keyframe markers as point sprites, with fewer draw calls and state changes.

Based on the builtin shader for round points with anti-aliased outline. This one is more pointy.

source/blender/gpu/CMakeLists.txt

@@ -185,6 +185,8 @@ data_to_c_simple(shaders/gpu_shader_edges_overlay_simple_geom.glsl SRC)
 data_to_c_simple(shaders/gpu_shader_edges_overlay_frag.glsl SRC)
 data_to_c_simple(shaders/gpu_shader_text_vert.glsl SRC)
 data_to_c_simple(shaders/gpu_shader_text_frag.glsl SRC)
+data_to_c_simple(shaders/gpu_shader_keyframe_diamond_vert.glsl SRC)
+data_to_c_simple(shaders/gpu_shader_keyframe_diamond_frag.glsl SRC)
 
 data_to_c_simple(shaders/gpu_shader_geometry.glsl SRC)
 data_to_c_simple(shaders/gpu_shader_fire_frag.glsl SRC)