Improve UX when we run out of texture space

Rather than leaving the frame un-rendered, this commit arranges
to make one last pass but with all image quad assignments skipped.

This should at least make a reasonable effort at displaying text
on the screen.

refs: https://github.com/wez/wezterm/issues/879

wezterm-gui/src/termwindow/mod.rs

@@ -223,6 +223,9 @@ pub struct TermWindow {
 
     event_states: HashMap<String, EventState>,
     has_animation: RefCell<Option<Instant>>,
+    /// We use this to attempt to do something reasonable
+    /// if we run out of texture space
+    allow_images: bool,
     scheduled_animation: RefCell<Option<Instant>>,
 
     gl: Option<Rc<glium::backend::Context>>,
@@ -483,6 +486,7 @@ impl TermWindow {
             event_states: HashMap::new(),
             has_animation: RefCell::new(None),
             scheduled_animation: RefCell::new(None),
+            allow_images: true,
         };
 
         let tw = Rc::new(RefCell::new(myself));

wezterm-gui/src/termwindow/render.rs

@@ -86,6 +86,8 @@ impl super::TermWindow {
         // If nothing on screen needs animating, then we can avoid
         // invalidating as frequently
         *self.has_animation.borrow_mut() = None;
+        // Start with the assumption that we should allow images to render
+        self.allow_images = true;
 
         let start = Instant::now();
 
@@ -138,6 +140,14 @@ impl super::TermWindow {
                         };
 
                         if let Err(err) = result {
+                            if self.allow_images {
+                                self.allow_images = false;
+                                log::info!(
+                                    "Not enough texture space ({:#}); \
+                                     will retry render with images disabled",
+                                    err
+                                );
+                            } else {
                                 log::error!(
                                     "Failed to {} texture: {}",
                                     if pass == 0 { "clear" } else { "resize" },
@@ -145,6 +155,7 @@ impl super::TermWindow {
                                 );
                                 break;
                             }
+                        }
                     } else if err.root_cause().downcast_ref::<ClearShapeCache>().is_some() {
                         self.shape_cache.borrow_mut().clear();
                     } else {
@@ -260,7 +271,7 @@ impl super::TermWindow {
             },
         );
 
-        if pos.index == 0 {
+        if pos.index == 0 && self.allow_images {
             // Render the window background image
             if let Some(im) = self.window_background.as_ref() {
                 let mut quad = quads.allocate()?;
@@ -1214,6 +1225,10 @@ impl super::TermWindow {
         hsv: Option<config::HsbTransform>,
         glyph_color: LinearRgba,
     ) -> anyhow::Result<()> {
+        if !self.allow_images {
+            return Ok(());
+        }
+
         let padding = self
             .render_metrics
             .cell_size