Revert "internal/graphicsdriver/opengl, metal, directx: skip multiplying a scale when a color matrix is used"

This reverts commit b457dc3307.

Reason: a color scale might be used with ColorM (DrawTriangles)

internal/graphicsdriver/directx/pipeline_windows.go

@@ -221,7 +221,8 @@ float4 PSMain(PSInput input) : SV_TARGET {
   color = mul(color_matrix_body, color) + color_matrix_translation;
   // Premultiply alpha
   color.rgb *= color.a;
-  // Do not apply the color scale as the scale should always be (1, 1, 1, 1) when a color matrix is used.
+  // Apply color scale.
+  color *= input.color;
   // Clamp the output.
   color.rgb = min(color.rgb, color.a);
   return color;

internal/graphicsdriver/metal/graphics_darwin.go

@@ -197,6 +197,7 @@ struct FragmentShaderImpl {
       c.rgb /= c.a + (1.0 - sign(c.a));
       c = (color_matrix_body * c) + color_matrix_translation;
       c.rgb *= c.a;
+      c *= v.color;
       c.rgb = min(c.rgb, c.a);
     } else {
       c *= v.color;

internal/graphicsdriver/opengl/defaultshader.go

@@ -237,12 +237,14 @@ void main(void) {
   color = (color_matrix_body * color) + color_matrix_translation;
   // Premultiply alpha
   color.rgb *= color.a;
-  // Do not apply the color scale as the scale should always be (1, 1, 1, 1) when a color matrix is used.
+  // Apply color scale.
+  color *= varying_color_scale;
   // Clamp the output.
   color.rgb = min(color.rgb, color.a);
 # else
-  // Apply the color scale.
+  // Apply color scale.
   color *= varying_color_scale;
+  // No clamping needed as the color matrix shader is used then.
 # endif  // defined(USE_COLOR_MATRIX)
 
   gl_FragColor = color;