Add an example demonstrating the basics of occlusion queries (#1548)

* Add an example demonstrating the basics of occlusion queries

* Make some functions const

Author: Rua

Diff for: CHANGELOG_VULKANO.md

@@ -31,6 +31,7 @@
   - `get_results` has been added to `QueriesRange`, to copy query results to the CPU.
   - The following functions have been added to both `SyncCommandBufferBuilder` and `AutoCommandBufferBuilder`: `begin_query` (still unsafe), `end_query` (safe), `write_timestamp` (still unsafe), `copy_query_pool_results` (safe), `reset_command_pool` (still unsafe).
   - Better documentation of everything in the `query` module.
+  - An example demonstrating occlusion queries.
 - The deprecated `cause` trait function on Vulkano error types is replaced with `source`.
 - Vulkano-shaders: Fixed and refined the generation of the `readonly` descriptor attribute. It should now correctly mark uniforms and sampled images as read-only, but storage buffers and images only if explicitly marked as `readonly` in the shader.
 - Fixed bug in descriptor array layers check when the image is a cubemap.