feat: default projection mapping with shift + M

default/inc_map.glsl

@@ -0,0 +1,26 @@
+#ifndef INC_MAP
+#define INC_MAP
+
+vec2 project_4p(vec2 uv, vec2 p1, vec2 p2, vec2 p3, vec2 p4) {
+    float k1 = (p4.y - p3.y) / (p2.y - p3.y);
+    float k2 = (p3.y - p1.y) / (p2.y - p3.y);
+    float a = (p4.x - p3.x + k1 * p3.x - k1 * p2.x) / (p1.x - p3.x - k2 * p3.x + k2 * p2.x);
+    float b = k1 + a * k2;
+    float c = 1 - a - b;
+    // https://math.stackexchange.com/questions/296794/finding-the-transform-matrix-from-4-projected-points-with-javascript/339033#339033
+    mat3 projection_a = mat3(
+            a * p1.x, a * p1.y, a,
+            b * p2.x, b * p2.y, b,
+            c * p3.x, c * p3.y, c
+        );
+    mat3 projection_a_prime = inverse(projection_a);
+    mat3 projection_b = mat3(
+            0, 0, -1,
+            1, 0, 1,
+            0, 1, 1
+        );
+    vec3 tmp = projection_b * projection_a_prime * vec3(uv, 1);
+    return tmp.xy / tmp.z;
+}
+
+#endif