Auto merge of #3404 - kvark:alloc-zero, r=gw3583

Fix handling of zero regions by texture allocator

Fixes https://bugzilla.mozilla.org/show_bug.cgi?id=1513017

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webrender/src/texture_allocator.rs

@@ -215,6 +215,13 @@ fn random_fill(count: usize, texture_size: i32) -> f32 {
     );
     let mut rng = thread_rng();
     let mut allocator = ArrayAllocationTracker::new();
+
+    // check for empty allocation
+    assert_eq!(
+        allocator.allocate(&DeviceIntSize::new(0, 12)),
+        Some((FreeRectSlice(0), DeviceIntPoint::zero())),
+    );
+
     let mut slices: Vec<Vec<DeviceIntRect>> = Vec::new();
     let mut requested_area = 0f32;
     // fill up the allocator

webrender/src/tiling.rs

@@ -243,6 +243,11 @@ impl<T: RenderTarget> RenderTargetList<T> {
             }
         };
 
+        if alloc_size.is_empty_or_negative() && self.targets.is_empty() {
+            // push an unused target here, only if we don't have any
+            self.targets.push(T::new(self.screen_size));
+        }
+
         self.targets[free_rect_slice.0 as usize]
             .add_used(DeviceIntRect::new(origin, alloc_size));