fix(glsl): fix glsl builtin mix function

Author: MrTelanie

src/glsl/builtin.js

@@ -167,7 +167,7 @@ export class BuiltIn {
 
   mix(x, y, alpha) {
     return fastCalc(
-      (x, y, alpha) => alpha * x + (1 - alpha) * y,
+      (x, y, alpha) => (1 - alpha) * x + alpha * y,
       x, y, alpha
     );
   }

test/glsl/sim.js

@@ -293,7 +293,7 @@ float action(vec2 one, vec2 two) {
   it('works when using mix.', () => {
     const shader = ({ mix, vec2 }) => {
       let bar = vec2(() => {
-        return mix(vec2(1.0, 0.0), vec2(3.0, 6.0), 0.5);
+        return mix(vec2(1.0, 0.0), vec2(4.0, 6.0), 0.75);
       });
       return { bar };
     };
@@ -302,8 +302,8 @@ float action(vec2 one, vec2 two) {
     const { bar } = js;
 
     const result = bar();
-    assert.equal(result.x, 2);
-    assert.equal(result.y, 3);
+    assert.equal(result.x, 3.25);
+    assert.equal(result.y, 4.5);
   });
 
   it('works fine with sampler2D from array buffer.', () => {