feat(terrain): holes cut in the heightmap terrain and shader grass painted over it

shaders/grass.frag

@@ -0,0 +1,41 @@
+// grass of the heightmap terrain (opengl46_grass, grass.vert): the blades lit like the rest of the scene
+
+in vec3 f_normal;
+in vec4 f_pos;
+in vec3 f_albedo;
+in vec4 f_clip_pos;
+in vec4 f_clip_future_pos;
+
+#include <common>
+
+layout(location = 0) out vec4 out_color;
+#if MOTIONBLUR_ENABLED
+layout(location = 1) out vec4 out_motion;
+#endif
+
+#include <light_common.glsl>
+#include <apply_fog.glsl>
+#include <tonemapping.glsl>
+
+void main()
+{
+	vec3 fragnormal = normalize(f_normal);
+	glossiness = 6.0;
+	metalic = 0.0;
+	vec3 fragcolor = apply_lights(ambient, fragnormal, f_albedo, 0.0, 0.2, shadow_tone);
+
+	vec4 color = vec4(apply_fog(fragcolor), 1.0);
+#if POSTFX_ENABLED
+	out_color = color;
+#else
+	out_color = tonemap(color);
+#endif
+#if MOTIONBLUR_ENABLED
+	{
+		vec2 a = (f_clip_future_pos.xy / f_clip_future_pos.w) * 0.5 + 0.5;
+		vec2 b = (f_clip_pos.xy / f_clip_pos.w) * 0.5 + 0.5;
+
+		out_motion = vec4(a - b, 0.0f, 0.0f);
+	}
+#endif
+}