qemu: enable support for CGL backend

Thanks to @startergo for the changes. This should enable OpenGL 4.1 on Linux

Resolves #4285

Author: osy

Configuration/UTMQemuConfiguration+Arguments.swift

@@ -201,7 +201,7 @@ import Virtualization // for getting network interfaces
         } else {
             "streaming-video=filter"
         }
-        "gl=\(isGLSupported && !isRemoteSpice ? "on" : "off")"
+        "gl=\(glBackend)"
         f()
         f("-chardev")
         if isRemoteSpice {
@@ -281,7 +281,28 @@ import Virtualization // for getting network interfaces
             display.hardware.rawValue.contains("-gl-") || display.hardware.rawValue.hasSuffix("-gl")
         }
     }
-    
+
+    private var rendererBackend: UTMQEMURendererBackend {
+        let rawValue = UserDefaults.standard.integer(forKey: "QEMURendererBackend")
+        return UTMQEMURendererBackend(rawValue: rawValue) ?? .qemuRendererBackendDefault
+    }
+
+    private var glBackend: String {
+        if isGLSupported && !isRemoteSpice {
+            #if os(macOS)
+            if rendererBackend == .qemuRendererBackendCGL {
+                return "core"
+            } else {
+                return "es"
+            }
+            #else
+            return "on"
+            #endif
+        } else {
+            return "off"
+        }
+    }
+
     private var isSparc: Bool {
         system.architecture == .sparc || system.architecture == .sparc64
     }

Platform/macOS/SettingsView.swift

@@ -232,6 +232,7 @@ struct DisplaySettingsView: View {
                     Text("Default").tag(UTMQEMURendererBackend.qemuRendererBackendDefault)
                     Text("ANGLE (OpenGL)").tag(UTMQEMURendererBackend.qemuRendererBackendAngleGL)
                     Text("ANGLE (Metal)").tag(UTMQEMURendererBackend.qemuRendererBackendAngleMetal)
+                    Text("Apple Core OpenGL").tag(UTMQEMURendererBackend.qemuRendererBackendCGL)
                 }.help("By default, the best renderer for this device will be used. You can override this with to always use a specific renderer. This only applies to QEMU VMs with GPU accelerated graphics.")
                 HStack {
                     Stepper("FPS Limit", value: $qemuRendererFpsLimit, in: 0...240, step: 15)

Services/UTMQemuSystemBackends.h

@@ -22,7 +22,8 @@ typedef NS_ENUM(NSInteger, UTMQEMURendererBackend) {
     kQEMURendererBackendDefault = 0,
     kQEMURendererBackendAngleGL = 1,
     kQEMURendererBackendAngleMetal = 2,
-    kQEMURendererBackendMax = 3,
+    kQEMURendererBackendCGL = 3,
+    kQEMURendererBackendMax = 4,
 };
 
 /// Specify the sound backend for this VM