metal: Add hint to select low power device instead of the default one (#8182)

On some system like MacBook Pro Intel with AMD card, asking for the default device will always return the AMD GPU.
This is not an issue for 99% of the case when the renderer context is here to provide the maximum performance level like for game.
However, for video application using GPU for 1 quad and 1 texture, using the discrete GPU for that lead to an important power consumption (4 to 8W), heat increase, and fan noise.
With this patch, I successfully amend ffplay to only use the integrated GPU (i.e. the Intel one), instead of the discrete GPU (i.e. the AMD one).

include/SDL_hints.h

@@ -1474,6 +1474,17 @@ extern "C" {
  */
 #define SDL_HINT_RENDER_VSYNC               "SDL_RENDER_VSYNC"
 
+/**
+ *  \brief  A variable controlling whether the Metal render driver select low power device over default one
+ *
+ *  This variable can be set to the following values:
+ *    "0"       - Use the prefered OS device
+ *    "1"       - Select a low power one
+ *
+ *  By default the prefered OS device is used.
+ */
+#define SDL_HINT_RENDER_METAL_PREFER_LOW_POWER_DEVICE "SDL_RENDER_METAL_PREFER_LOW_POWER_DEVICE"
+
 /**
  *  \brief  A variable controlling if VSYNC is automatically disable if doesn't reach the enough FPS
  *