Auto merge of #1768 - glennw:fix-overflow, r=nical

Cull primitives with invalid dimensions, to avoid overflow later.

Primitives with invalid dimensions can cause overflow and general
badness in the texture and render task allocators. Detect them
and ensure they are not considered visible.

Fixes servo/servo#18655

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Author: bors-servo

webrender/src/prim_store.rs

@@ -1075,6 +1075,12 @@ impl PrimitiveStore {
             let metadata = &mut self.cpu_metadata[prim_index.0];
             metadata.screen_rect = None;
 
+            if metadata.local_rect.size.width <= 0.0 ||
+               metadata.local_rect.size.height <= 0.0 {
+                warn!("invalid primitive rect {:?}", metadata.local_rect);
+                return None;
+            }
+
             if !metadata.is_backface_visible &&
                prim_context.packed_layer.transform.is_backface_visible() {
                 return None;