UI: Change the default color setting in the UI from sRGB to 709

It seems like YouTube applies nonlinear-to-linear sRGB, and
linear-to nonlinear-709 transformations to uploaded videos now. This
makes sRGB too dark on their platform for video players that alias 709
as sRGB, which is almost everyone. Make 709 the default to keep peace.

UI/window-basic-main.cpp

@@ -1392,7 +1392,7 @@ bool OBSBasic::InitBasicConfigDefaults()
 	config_set_default_uint(basicConfig, "Video", "FPSDen", 1);
 	config_set_default_string(basicConfig, "Video", "ScaleType", "bicubic");
 	config_set_default_string(basicConfig, "Video", "ColorFormat", "NV12");
-	config_set_default_string(basicConfig, "Video", "ColorSpace", "sRGB");
+	config_set_default_string(basicConfig, "Video", "ColorSpace", "709");
 	config_set_default_string(basicConfig, "Video", "ColorRange",
 				  "Partial");