UI: Add ini option to use different graphics adapters

Allows setting a specific graphics adapter index to use for Direct3D 11.
This is currently meant to be sort of "undocumented" in implementing it
this way due to the fact that users will be guaranteed to break their
configurations if this is implemented in the UI (even if in advanced
settings).

UI/window-basic-main.cpp

@@ -2921,7 +2921,8 @@ int OBSBasic::ResetVideo()
 		VIDEO_CS_601 : VIDEO_CS_709;
 	ovi.range          = astrcmpi(colorRange, "Full") == 0 ?
 		VIDEO_RANGE_FULL : VIDEO_RANGE_PARTIAL;
-	ovi.adapter        = 0;
+	ovi.adapter        = config_get_uint(App()->GlobalConfig(),
+			"Video", "AdapterIdx");
 	ovi.gpu_conversion = true;
 	ovi.scale_type     = GetScaleType(basicConfig);