Provide a matrix inverse shim for GLES 2.0.

Author: John O'Neil

impeller/compiler/shader_lib/impeller/gradient.glsl

@@ -6,13 +6,14 @@
 #define GRADIENT_GLSL_
 
 #include <impeller/texture.glsl>
+#include <impeller/transform.glsl>
 
 mat3 IPMapToUnitX(vec2 p0, vec2 p1) {
   // Returns a matrix that maps [p0, p1] to [(0, 0), (1, 0)]. Results are
   // undefined if p0 = p1.
   return mat3(0.0, -1.0, 0.0, 1.0, 0.0, 0.0, 0.0, 0.0, 1.0) *
-         inverse(mat3(p1.y - p0.y, p0.x - p1.x, 0.0, p1.x - p0.x, p1.y - p0.y,
-                      0.0, p0.x, p0.y, 1.0));
+         IPMat3Inverse(mat3(p1.y - p0.y, p0.x - p1.x, 0.0, p1.x - p0.x,
+                            p1.y - p0.y, 0.0, p0.x, p0.y, 1.0));
 }
 
 /// Compute the t value for a conical gradient at point `p` between the 2

impeller/compiler/shader_lib/impeller/transform.glsl

@@ -26,4 +26,33 @@ vec4 IPPositionForGlyphPosition(mat4 mvp,
               unit_position.y * destination_size.y, 0.0, 1.0);
 }
 
-#endif
+#ifdef IMPELLER_TARGET_OPENGLES
+
+// Shim matrix `inverse` for versions that lack it.
+// TODO: This could be gated on GLSL < 1.4.
+mat3 IPMat3Inverse(mat3 m) {
+  float a00 = m[0][0], a01 = m[0][1], a02 = m[0][2];
+  float a10 = m[1][0], a11 = m[1][1], a12 = m[1][2];
+  float a20 = m[2][0], a21 = m[2][1], a22 = m[2][2];
+
+  float b01 = a22 * a11 - a12 * a21;
+  float b11 = -a22 * a10 + a12 * a20;
+  float b21 = a21 * a10 - a11 * a20;
+
+  float det = a00 * b01 + a01 * b11 + a02 * b21;
+
+  return mat3(b01, (-a22 * a01 + a02 * a21), (a12 * a01 - a02 * a11), b11,
+              (a22 * a00 - a02 * a20), (-a12 * a00 + a02 * a10), b21,
+              (-a21 * a00 + a01 * a20), (a11 * a00 - a01 * a10)) /
+         det;
+}
+
+#else  // IMPELLER_TARGET_OPENGLES
+
+mat3 IPMat3Inverse(mat3 m) {
+  return inverse(m);
+}
+
+#endif  // IMPELLER_TARGET_OPENGLES
+
+#endif  // TRANSFORM_GLSL_