Make sure egui can handle zero-sized screen rect

This would previously hit a debug assert

Fixes https://github.com/emilk/egui/issues/395

egui_demo_lib/src/lib.rs

@@ -139,3 +139,22 @@ fn test_egui_e2e() {
         assert!(!clipped_meshes.is_empty());
     }
 }
+
+#[test]
+fn test_egui_zero_window_size() {
+    let mut demo_windows = crate::DemoWindows::default();
+    let mut ctx = egui::CtxRef::default();
+    let raw_input = egui::RawInput {
+        screen_rect: Some(egui::Rect::from_min_max(egui::Pos2::ZERO, egui::Pos2::ZERO)),
+        ..Default::default()
+    };
+
+    const NUM_FRAMES: usize = 5;
+    for _ in 0..NUM_FRAMES {
+        ctx.begin_frame(raw_input.clone());
+        demo_windows.ui(&ctx);
+        let (_output, shapes) = ctx.end_frame();
+        let clipped_meshes = ctx.tessellate(shapes);
+        assert!(clipped_meshes.is_empty(), "There should be nothing to show");
+    }
+}