UI: Use default scaled res. of 1280x720 or equiv.

Originally this value defaulted to 1.5 downscaling, but on very high
resolution displays this would cause the default to be above 1280x720,
which is not ideal for streaming/recording due to the CPU usage
requirements.

Instead, it will now find the closest resolution with a pixel count
equivalent to or closest below 1280x720, and use that instead.

obs/window-basic-main.cpp

@@ -531,6 +531,22 @@ bool OBSBasic::InitService()
 	return true;
 }
 
+static const double scaled_vals[] =
+{
+	1.0,
+	1.25,
+	(1.0/0.75),
+	1.5,
+	(1.0/0.6),
+	1.75,
+	2.0,
+	2.25,
+	2.5,
+	2.75,
+	3.0,
+	0.0
+};
+
 bool OBSBasic::InitBasicConfigDefaults()
 {
 	vector<MonitorInfo> monitors;
@@ -618,10 +634,20 @@ bool OBSBasic::InitBasicConfigDefaults()
 	config_set_default_uint  (basicConfig, "Video", "BaseCX",   cx);
 	config_set_default_uint  (basicConfig, "Video", "BaseCY",   cy);
 
-	cx = cx * 10 / 15;
-	cy = cy * 10 / 15;
-	config_set_default_uint  (basicConfig, "Video", "OutputCX", cx);
-	config_set_default_uint  (basicConfig, "Video", "OutputCY", cy);
+	int i = 0;
+	uint32_t scale_cx = cx;
+	uint32_t scale_cy = cy;
+
+	/* use a default scaled resolution that has a pixel count no higher
+	 * than 1280x720 */
+	while (((scale_cx * scale_cy) > (1280 * 720)) && scaled_vals[i] > 0.0) {
+		double scale = scaled_vals[i++];
+		scale_cx = uint32_t(double(cx) / scale);
+		scale_cy = uint32_t(double(cy) / scale);
+	}
+
+	config_set_default_uint  (basicConfig, "Video", "OutputCX", scale_cx);
+	config_set_default_uint  (basicConfig, "Video", "OutputCY", scale_cy);
 
 	config_set_default_uint  (basicConfig, "Video", "FPSType", 0);
 	config_set_default_string(basicConfig, "Video", "FPSCommon", "30");