render: skip clearing images if they aren't supported

examples/nvim.zig

@@ -33,7 +33,7 @@ pub fn main() !void {
     defer loop.stop();
 
     // Optionally enter the alternate screen
-    try vx.enterAltScreen();
+    // try vx.enterAltScreen();
 
     var nvim = try vaxis.widgets.nvim.Nvim(Event).init(alloc, &loop);
     try nvim.spawn();

src/Vaxis.zig

@@ -251,6 +251,7 @@ pub fn render(self: *Vaxis) !void {
     } = .{};
 
     // Clear all images
+    if (self.caps.kitty_graphics)
         _ = try tty.write(ctlseqs.kitty_graphics_clear);
 
     var i: usize = 0;