[core] Added BlendColor GL config value

include/mbgl/gl/gl_values.hpp

@@ -214,6 +214,19 @@ constexpr bool operator!=(const BlendFunc::Type& a, const BlendFunc::Type& b) {
     return a.sfactor != b.sfactor || a.dfactor != b.dfactor;
 }
 
+struct BlendColor {
+    using Type = Color;
+    static const Type Default;
+    inline static void Set(const Type& value) {
+        MBGL_CHECK_ERROR(glBlendColor(value.r, value.g, value.b, value.a));
+    }
+    inline static Type Get() {
+        GLfloat floats[4];
+        MBGL_CHECK_ERROR(glGetFloatv(GL_BLEND_COLOR, floats));
+        return { floats[0], floats[1], floats[2], floats[3] };
+    }
+};
+
 struct Program {
     using Type = GLuint;
     static const Type Default;

src/mbgl/gl/gl_config.cpp

@@ -13,6 +13,7 @@ const DepthTest::Type DepthTest::Default = GL_FALSE;
 const DepthFunc::Type DepthFunc::Default = GL_LEQUAL;
 const Blend::Type Blend::Default = GL_TRUE;
 const BlendFunc::Type BlendFunc::Default = { GL_ONE, GL_ONE_MINUS_SRC_ALPHA };
+const BlendColor::Type BlendColor::Default = { 0, 0, 0, 0 };
 const ColorMask::Type ColorMask::Default = { GL_TRUE, GL_TRUE, GL_TRUE, GL_TRUE };
 const ClearDepth::Type ClearDepth::Default = 1;
 const ClearColor::Type ClearColor::Default = { 0, 0, 0, 0 };

src/mbgl/gl/gl_config.hpp

@@ -56,6 +56,7 @@ public:
         depthFunc.reset();
         blend.reset();
         blendFunc.reset();
+        blendColor.reset();
         colorMask.reset();
         clearDepth.reset();
         clearColor.reset();
@@ -80,6 +81,7 @@ public:
         depthFunc.setDirty();
         blend.setDirty();
         blendFunc.setDirty();
+        blendColor.setDirty();
         colorMask.setDirty();
         clearDepth.setDirty();
         clearColor.setDirty();
@@ -103,6 +105,7 @@ public:
     Value<DepthFunc> depthFunc;
     Value<Blend> blend;
     Value<BlendFunc> blendFunc;
+    Value<BlendColor> blendColor;
     Value<ColorMask> colorMask;
     Value<ClearDepth> clearDepth;
     Value<ClearColor> clearColor;