Fix weird text layout behaviour with GtkBackend, never tell Pango the desired height

Might want to change this behaviour in future but for now it lines up with
AppKitBackend's text layout anyway. And it prevents weird behaviour where
Pango says the text will be one line but Gtk renders it as multiple.

Author: stackotter

Sources/GtkBackend/GtkBackend.swift

@@ -293,7 +293,7 @@ public final class GtkBackend: AppBackend {
         let (width, height) = pango.getTextSize(
             text,
             proposedWidth: (proposedFrame?.x).map(Double.init),
-            proposedHeight: (proposedFrame?.y).map(Double.init)
+            proposedHeight: nil
         )
         return SIMD2(width, height)
     }