new collision displacer: got one-block-ceiling-passthru bug, but otherwise tolerable

assets/quickgreyscale.frag

@@ -0,0 +1,16 @@
+#ifdef GL_ES
+    precision mediump float;
+#endif
+
+varying vec4 v_color;
+varying vec2 v_texCoords;
+uniform sampler2D u_texture;
+uniform mat4 u_projTrans;
+
+void main() {
+        vec3 color = texture2D(u_texture, v_texCoords).rgb;
+        float gray = (3.0 * color.r + 4.0 * color.g + color.b) / 8.0;
+        vec3 grayscale = vec3(gray);
+
+        gl_FragColor = vec4(grayscale, 1.0);
+}