GL backend: Avoid run-time opengl errors with clipped zero width or height rectangles

If a Rectangle has a border-radius and clipping, we use an FBO to render
the children and then use femtovg's stencil clipping. If the Rectangle
has a zero width or height, we would end up trying to create a texture
with such dimensions, which produces run-time opengl errors.

We can detect this situation and avoid it early on. The same might happen for shadows.

Fixes #377

sixtyfps_runtime/rendering_backends/gl/images.rs

@@ -139,7 +139,10 @@ impl CachedImage {
         Self(RefCell::new(Texture { id: image_id, canvas: canvas.clone() }.into()))
     }
 
-    pub fn new_empty_on_gpu(canvas: &CanvasRc, width: usize, height: usize) -> Self {
+    pub fn new_empty_on_gpu(canvas: &CanvasRc, width: usize, height: usize) -> Option<Self> {
+        if width == 0 || height == 0 {
+            return None;
+        }
         let image_id = canvas
             .borrow_mut()
             .create_image_empty(
@@ -149,7 +152,7 @@ impl CachedImage {
                 femtovg::ImageFlags::PREMULTIPLIED | femtovg::ImageFlags::FLIP_Y,
             )
             .unwrap();
-        Self::new_on_gpu(canvas, image_id)
+        Self::new_on_gpu(canvas, image_id).into()
     }
 
     #[cfg(feature = "svg")]
@@ -352,6 +355,9 @@ impl CachedImage {
             &canvas,
             size.width.ceil() as usize,
             size.height.ceil() as usize,
+        )
+        .expect(
+            "internal error: this can only fail if the filtered image was zero width or height",
         );
 
         let filtered_image_id = match &*filtered_image.0.borrow() {

sixtyfps_runtime/rendering_backends/gl/lib.rs

@@ -934,7 +934,7 @@ impl ItemRenderer for GLItemRenderer {
             return;
         }
 
-        let cache_entry = self
+        let cache_entry = match self
             .graphics_window
             .graphics_cache
             .borrow_mut()
@@ -961,7 +961,7 @@ impl ItemRenderer for GLItemRenderer {
                         &self.shared_data.canvas,
                         shadow_image_width,
                         shadow_image_height,
-                    );
+                    )?;
 
                     {
                         let mut canvas = self.shared_data.canvas.borrow_mut();
@@ -1033,8 +1033,10 @@ impl ItemRenderer for GLItemRenderer {
                     Rc::new(shadow_image)
                 })
                 .into()
-            })
+            }) {
-            .expect("internal error: creation of the cached shadow image must always succeed");
+            Some(cached_shadow_image) => cached_shadow_image,
+            None => return, // Zero width or height shadow
+        };
 
         let shadow_image = cache_entry.as_image();
 
@@ -1514,11 +1516,15 @@ impl GLItemRenderer {
         let layer_width = path_bounds.maxx - path_bounds.minx;
         let layer_height = path_bounds.maxy - path_bounds.miny;
 
-        let clip_buffer_img = CachedImage::new_empty_on_gpu(
+        let clip_buffer_img = match CachedImage::new_empty_on_gpu(
             &self.shared_data.canvas,
             layer_width as _,
             layer_height as _,
-        );
+        ) {
+            Some(clip_buffer) => clip_buffer,
+            None => return, // Zero width or height clip path
+        };
+
         {
             let mut canvas = self.shared_data.canvas.borrow_mut();