add(continous): example which provides a fixed render schedule

The other examples did rendering based on events, which this renderer
does not. This makes these applications potentially not that efficient,
but allows for consistent frame times that make animations, etc.
possible. This example serves to show that you can use `zterm` for both
types of render scheduling and even change between them without much
efford.

src/queue.zig

@@ -90,6 +90,23 @@ pub fn Queue(comptime T: type, comptime size: usize) type {
             assert(!this.isEmptyLH());
         }
 
+        pub fn lock(this: *QueueType) void {
+            this.mutex.lock();
+        }
+
+        pub fn unlock(this: *QueueType) void {
+            this.mutex.unlock();
+        }
+
+        /// Used to efficiently drain the queue
+        pub fn drain(this: *QueueType) ?T {
+            if (this.isEmptyLH()) return null;
+
+            const result = this.buf[this.mask(this.read_index)];
+            this.read_index = this.mask2(this.read_index + 1);
+            return result;
+        }
+
         fn isEmptyLH(this: QueueType) bool {
             return this.write_index == this.read_index;
         }
@@ -114,7 +131,7 @@ pub fn Queue(comptime T: type, comptime size: usize) type {
         }
 
         /// Returns the length
-        fn len(this: QueueType) usize {
+        pub fn len(this: QueueType) usize {
             const wrap_offset = 2 * this.buf.len *
                 @intFromBool(this.write_index < this.read_index);
             const adjusted_write_index = this.write_index + wrap_offset;