carla-plugin: force qt to use xcb backend if going to embed UI

Signed-off-by: falkTX <falktx@falktx.com>

source/frontend/carla-plugin

@@ -578,17 +578,17 @@ class CarlaEmbedW(QEmbedWidget):
 
 if __name__ == '__main__':
     # -------------------------------------------------------------
-    # App initialization
-
-    app = CarlaApplication("Carla2-Plugin")
+    # Get details regarding target usage
 
-    # -------------------------------------------------------------
-    # Set-up custom signal handling
+    try:
+        winId = int(os.getenv("CARLA_PLUGIN_EMBED_WINID"))
+    except:
+        winId = 0
 
-    setUpSignals()
+    usingEmbed = bool(LINUX and winId != 0)
 
     # -------------------------------------------------------------
-    # Init host backend
+    # Init host backend (part 1)
 
     isPatchbay = sys.argv[0].rsplit(os.path.sep)[-1].lower().replace(".exe","") == "carla-plugin-patchbay"
 
@@ -596,19 +596,34 @@ if __name__ == '__main__':
     host.processMode       = ENGINE_PROCESS_MODE_PATCHBAY if isPatchbay else ENGINE_PROCESS_MODE_CONTINUOUS_RACK
     host.processModeForced = True
     host.nextProcessMode   = host.processMode
-    loadHostSettings(host)
 
     # -------------------------------------------------------------
-    # Create GUI
-
-    try:
-        winId = int(os.getenv("CARLA_PLUGIN_EMBED_WINID"))
-    except:
-        winId = 0
+    # Set-up environment
 
     gCarla.utils.setenv("CARLA_PLUGIN_EMBED_WINID", "0")
 
-    if LINUX and winId != 0:
+    if usingEmbed:
+        gCarla.utils.setenv("QT_QPA_PLATFORM", "xcb")
+
+    # -------------------------------------------------------------
+    # App initialization
+
+    app = CarlaApplication("Carla2-Plugin")
+
+    # -------------------------------------------------------------
+    # Set-up custom signal handling
+
+    setUpSignals()
+
+    # -------------------------------------------------------------
+    # Init host backend (part 2)
+
+    loadHostSettings(host)
+
+    # -------------------------------------------------------------
+    # Create GUI
+
+    if usingEmbed:
         gui = CarlaEmbedW(host, winId, isPatchbay)
     else:
         gui = CarlaMiniW(host, isPatchbay)