fix(eficore): don't panic when filling a rectangle past the right edge

The slice start was after its end when the rectangle began beyond the
framebuffer width, which happens when the cursor is at the right edge.

crates/eficore/src/framebuffer.rs

@@ -46,8 +46,11 @@ impl Framebuffer {
     pub fn fill_rect(&mut self, x: usize, y: usize, width: usize, height: usize, color: BltPixel) {
         let end_x = x.saturating_add(width).min(self.width);
         let end_y = y.saturating_add(height).min(self.height);
+        if x >= end_x {
+            return;
+        }
         for row in y..end_y {
-            // The row is in bounds, so its range is within the pixels.
+            // The row and columns are in bounds, so its range is within the pixels.
             self.pixels[row * self.width + x..row * self.width + end_x].fill(color);
         }
     }