Tesselator: ignore non-positive clip rectangles

Closes https://github.com/emilk/egui/issues/328

emath/src/rect.rs

@@ -304,16 +304,19 @@ impl Rect {
     }
 
     /// `min.x <= max.x && min.y <= max.y`.
+    #[inline(always)]
     pub fn is_non_negative(&self) -> bool {
         self.min.x <= self.max.x && self.min.y <= self.max.y
     }
 
     /// True if all members are also finite.
+    #[inline(always)]
     pub fn is_finite(&self) -> bool {
         self.min.is_finite() && self.max.is_finite()
     }
 
     /// True if any member is NaN.
+    #[inline(always)]
     pub fn any_nan(self) -> bool {
         self.min.any_nan() || self.max.any_nan()
     }

epaint/src/tessellator.rs

@@ -742,6 +742,10 @@ pub fn tessellate_shapes(
     let mut clipped_meshes: Vec<ClippedMesh> = Vec::default();
 
     for ClippedShape(clip_rect, shape) in shapes {
+        if !clip_rect.is_non_negative() {
+            continue; // skip empty clip rectangles
+        }
+
         let start_new_mesh = match clipped_meshes.last() {
             None => true,
             Some(cm) => cm.0 != clip_rect || cm.1.texture_id != shape.texture_id(),