Fix invalid initial size for VST3 and CLAP X11 UIs

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

dgl/src/pugl.cpp

@@ -353,7 +353,7 @@ PuglStatus puglSetSizeAndDefault(PuglView* view, uint width, uint height)
 
 #ifdef DGL_USING_X11
     // workaround issues in fluxbox, see https://github.com/lv2/pugl/issues/118
-    if (view->impl->win)
+    if (view->impl->win && !view->parent)
     {
         view->sizeHints[PUGL_DEFAULT_SIZE].width = view->sizeHints[PUGL_DEFAULT_SIZE].height = 0;
     }

distrho/src/DistrhoUIVST3.cpp

@@ -889,6 +889,7 @@ private:
     {
         DISTRHO_SAFE_ASSERT_RETURN(fView != nullptr,);
         DISTRHO_SAFE_ASSERT_RETURN(fFrame != nullptr,);
+        DISTRHO_SAFE_ASSERT_RETURN(width != 0 && height != 0,);
 
        #ifdef DISTRHO_OS_MAC
         const double scaleFactor = fUI.getScaleFactor();