Shaders: support color alpha in FlatShader.

src/Shaders/FlatShader.frag

@@ -27,9 +27,9 @@
 #endif
 
 #ifdef EXPLICIT_UNIFORM_LOCATION
-layout(location = 1) uniform vec3 color;
+layout(location = 1) uniform vec4 color;
 #else
-uniform lowp vec3 color;
+uniform lowp vec4 color;
 #endif
 
 #ifdef NEW_GLSL
@@ -37,5 +37,5 @@ out lowp vec4 fragmentColor;
 #endif
 
 void main() {
-    fragmentColor = vec4(color, 1.0);
+    fragmentColor = color;
 }

src/Shaders/FlatShader.h

@@ -64,7 +64,7 @@ template<UnsignedInt dimensions> class MAGNUM_SHADERS_EXPORT FlatShader: public
          * @brief Set color
          * @return Pointer to self (for method chaining)
          */
-        FlatShader<dimensions>* setColor(const Color3<>& color) {
+        FlatShader<dimensions>* setColor(const Color4<>& color) {
             setUniform(colorUniform, color);
             return this;
         }