Interactive shadows (#14702)

Co-authored-by: Ian Bassi <12130714+ianalexis@users.noreply.github.com>
Co-authored-by: Ian Bassi <ian.bassi@outlook.com>

resources/shaders/110/printbed_shadow.fs

@@ -0,0 +1,40 @@
+#version 110
+
+// Draws the build-plate as a receiver of the same depth shadow map used for object/self shadows,
+// so the plate, objects, and self-shadows all come from one unified technique.
+uniform sampler2D shadow_map;
+uniform mat4 shadow_light_vp;
+uniform float shadow_intensity;
+uniform float shadow_map_texel;
+
+varying vec4 world_pos;
+
+// Fraction of the 5x5 PCF kernel occluded from the light. Matches the object shadow shader.
+float shadow_occlusion()
+{
+    vec4 lp = shadow_light_vp * world_pos;
+    vec3 proj = lp.xyz / lp.w;
+    proj = proj * 0.5 + 0.5;
+    if (proj.z > 1.0)
+        return 0.0;
+
+    // The plate is a pure receiver (never rendered into the shadow map), so a tiny constant
+    // bias for numerical safety is enough here.
+    float bias = 0.0004;
+    float sum = 0.0;
+    for (int x = -2; x <= 2; ++x) {
+        for (int y = -2; y <= 2; ++y) {
+            float closest = texture2D(shadow_map, proj.xy + vec2(float(x), float(y)) * shadow_map_texel).r;
+            sum += (proj.z - bias > closest) ? 1.0 : 0.0;
+        }
+    }
+    return sum / 25.0;
+}
+
+void main()
+{
+    float occ = shadow_occlusion();
+    if (occ <= 0.0)
+        discard;
+    gl_FragColor = vec4(0.0, 0.0, 0.0, shadow_intensity * occ);
+}