Avoid flickery erase in render_line

This also improves perf.  The issue was that the erase of the whole
line of the background may be observed by the X server when SHM is
enabled because we don't wait for the exposure/copy to the window
pixmap before updating the same row fractionally later.

Avoiding the unconditional erase and just painting the full cell
contents over means that there's no opportunity for a visible flash.

In addition, since we render the cells background individually, that
erase was not needed.

This should save us some work and take some load off the cpu.

src/xwin.rs

@@ -652,15 +652,6 @@ impl<'a> TerminalWindow<'a> {
             &term::color::ColorAttribute::Background,
         );
 
-        // Clear this dirty row
-        self.buffer_image.borrow_mut().clear_rect(
-            0,
-            y,
-            self.width as usize,
-            self.cell_height,
-            background_color.into(),
-        );
-
         // Break the line into clusters of cells with the same attributes
         let cell_clusters = line.cluster();
         for cluster in cell_clusters {
@@ -791,6 +782,15 @@ impl<'a> TerminalWindow<'a> {
             }
         }
 
+        // Clear anything remaining to the right of the line
+        self.buffer_image.borrow_mut().clear_rect(
+            x,
+            y,
+            self.width as usize - x as usize,
+            self.cell_height,
+            background_color.into(),
+        );
+
         // If we have SHM available, we can send up just this changed line
         match &*self.buffer_image.borrow() {
             &BufferImage::Shared(ref shm) => {