Add t.graphics.lowpower = true/false love.conf option (defaults to false).

Add love.graphics.isLowPowerPreferred.

It's currently hooked up in the vulkan backend.
#2127

src/modules/graphics/Graphics.cpp

@@ -44,6 +44,7 @@ namespace graphics
 {
 
 static bool gammaCorrect = false;
+static bool lowPowerPreferred = false;
 static bool debugMode = false;
 static bool debugModeQueried = false;
 
@@ -149,6 +150,16 @@ void setRenderers(const std::vector<Renderer> &renderers)
 	_renderers = renderers;
 }
 
+void setLowPowerPreferred(bool preferred)
+{
+	lowPowerPreferred = preferred;
+}
+
+bool isLowPowerPreferred()
+{
+	return lowPowerPreferred;
+}
+
 Graphics *Graphics::createInstance()
 {
 	Graphics *instance = Module::getInstance<Graphics>(M_GRAPHICS);