Test vector and matrix scalarization does not affect output

Vector and matrix scalarization was turned off because
it affects the rendering output. We need to turn it
back on in some form for Metal. This test is added to
check that rendering output is not affected.

Test adapted from WebGL CTS test
https://github.com/KhronosGroup/WebGL/pull/3214

Bug: angleproject:6306
Change-Id: Ic1db18920064726761d6168f9dd5111772028598
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3115013
Commit-Queue: Gregg Tavares <gman@chromium.org>
Reviewed-by: Kenneth Russell <kbr@chromium.org>

src/tests/gl_tests/GLSLTest.cpp

diff --git a/src/tests/gl_tests/GLSLTest.cpp b/src/tests/gl_tests/GLSLTest.cpp
index e55b859..fce6f34 100644
--- a/src/tests/gl_tests/GLSLTest.cpp
+++ b/src/tests/gl_tests/GLSLTest.cpp
@@ -13132,6 +13132,57 @@
     EXPECT_PIXEL_COLOR_EQ(0, 0, GLColor::green);
 }
 
+TEST_P(GLSLTest, VectorAndMatrixScalarizationDoesNotAffectRendering)
+{
+    ANGLE_SKIP_TEST_IF(IsWindows() && IsNVIDIA() && (IsOpenGL() || IsOpenGLES()));
+
+    constexpr char kFS[] = R"(
+precision mediump float;
+
+varying vec2 v_texCoord;
+
+float a = 0.;
+#define A 0.
+
+#define r(a)    mat2( cos( a + vec4(0,-1.5708,1.5708,0) ) )
+vec2 c;
+#define f(U,a)  ( c = (U) * r(a) , sin(10.*c.x) )
+
+void main() {
+    vec2 U = v_texCoord;
+
+    gl_FragColor = U.y > .5
+        ? vec4( f(U,a) , f(U*4.,a) , 0,1.0)   // top
+        : vec4( f(U,A) , f(U*4.,A) , 0,1.0);  // bottom
+}
+
+)";
+
+    ANGLE_GL_PROGRAM(program, essl1_shaders::vs::Texture2D(), kFS);
+    drawQuad(program, essl1_shaders::PositionAttrib(), 0.5f);
+
+    // Make sure we rendered something.
+    EXPECT_PIXEL_NE(0, 0, 0, 0, 0, 0);
+
+    // Comapare one line in top half to one line in bottom half.
+    int compareWidth  = getWindowWidth();
+    int compareHeight = getWindowHeight() / 4;
+
+    ASSERT_GE(compareWidth, 2);
+    ASSERT_GE(compareHeight, 2);
+
+    GLubyte pixelValue[4];
+    constexpr int tolerance = 12;
+
+    for (int x = 0; x < compareWidth; ++x)
+    {
+        glReadPixels(x, compareHeight, 1, 1, GL_RGBA, GL_UNSIGNED_BYTE, pixelValue);
+        EXPECT_PIXEL_NEAR(x, getWindowHeight() - compareHeight, pixelValue[0], pixelValue[1],
+                          pixelValue[2], pixelValue[3], tolerance);
+    }
+    EXPECT_GL_NO_ERROR();
+}
+
 }  // anonymous namespace
 
 ANGLE_INSTANTIATE_TEST_ES2_AND_ES3_AND(GLSLTest, WithDirectSPIRVGeneration(ES2_VULKAN()));