SDL/Window: set render scaling method to linear

This looks better than NN scaling (default);

src/sdl/window.cpp

@@ -54,6 +54,9 @@ window::window(const std::string& title,
 	// Set default blend mode to blend.
 	SDL_SetRenderDrawBlendMode(*this, SDL_BLENDMODE_BLEND);
 
+	// Use linear scaling when rendering, if applicable.
+	SDL_SetHint(SDL_HINT_RENDER_SCALE_QUALITY, "linear");
+
 	pixel_format_ = info.texture_formats[0];
 
 	fill(0,0,0);