[core] don't force GL state to be set when calling .reset()

We are using Value::reset() to change a piece of GL state to its default value. However, the current implementation always executes the GL call, even if our state tracking system knows that it's already at that value. The new implementation of Value::reset() now respects that, resulting in a lot fewer GL calls.

include/mbgl/gl/gl_values.hpp

@@ -140,6 +140,10 @@ struct StencilOp {
     }
 };
 
+constexpr bool operator!=(const StencilOp::Type& a, const StencilOp::Type& b) {
+    return a.sfail != b.sfail || a.dpfail != b.dpfail || a.dppass != b.dppass;
+}
+
 struct DepthRange {
     struct Type { GLfloat near, far; };
     static const Type Default;

src/mbgl/gl/gl_config.hpp

@@ -21,9 +21,7 @@ public:
     }
 
     void reset() {
-        dirty = true;
-        current = T::Default;
-        T::Set(current);
+        *this = T::Default;
     }
 
     void setDirty() {

src/mbgl/renderer/painter.cpp

@@ -83,6 +83,7 @@ Painter::Painter(const TransformState& state_,
     overdrawShader.circle = std::make_unique<CircleShader>(store, Shader::Overdraw);
 
     // Reset GL values
+    config.setDirty();
     config.reset();
 }
 

test/gl/object.cpp

@@ -18,11 +18,13 @@ static bool setFlag = false;
 
 struct MockGLObject {
     using Type = bool;
-    static const Type Default = false;
+    static const Type Default;
     static Type Get() { getFlag = true; return true; }
     static void Set(const Type&) { setFlag = true; }
 };
 
+const bool MockGLObject::Default = false;
+
 TEST(GLObject, Preserve) {
     getFlag = false;
     setFlag = false;
@@ -61,7 +63,7 @@ TEST(GLObject, Value) {
 
     object->reset();
     EXPECT_EQ(object->getCurrent(), false);
-    EXPECT_TRUE(object->getDirty());
+    EXPECT_FALSE(object->getDirty());
     EXPECT_TRUE(setFlag);
 }