Added love.graphics.origin(): resets the coordinate system (replaces the current transformation matrix with the identity)

--HG--
branch : love.graphics.origin

src/modules/graphics/opengl/Graphics.cpp

@@ -207,7 +207,6 @@ void Graphics::reset()
 void Graphics::clear()
 {
 	glClear(GL_COLOR_BUFFER_BIT);
-	glLoadIdentity();
 }
 
 void Graphics::present()
@@ -1117,6 +1116,11 @@ void Graphics::shear(float kx, float ky)
 	glMultMatrixf((const GLfloat *)t.getElements());
 }
 
+void Graphics::origin()
+{
+	glLoadIdentity();
+}
+
 bool Graphics::hasFocus() const
 {
 	return currentWindow->hasFocus();