Hello rectangle

Based on the second half of https://learnopengl.com/Getting-started/Hello-Triangle from Element Buffer Objects onward.

Press space to toggle between filled and wireframe pipelines.

Note that we need multiple definitions of the render pipeline with different polygon_mode's:

fill_pipeline uses PolygonMode::Fill
wireframe_pipeline uses PolygonMode::Line

Only POLYGON_MODE_FILL is enabled by default so the POLYGONE_MODE_LINE feature must be enabled explicitly.