This removes the old `DrawEngineType` and use the new `DrawEngine`
virtual class instead.
This removes a lot of boilerplate functions that were only there for
legacy reason.
To this end, some engines that were based on static functions have been
refactored into `Instance` classes. This was particularly cumbersome
for the Grease pencil engine which needed some more refactoring.
The `Engine` class that is in each namespace is a workaround to isolate
the internal implementation (i.e. the `Instance`) to the engine
modules. Without this, the whole engine is getting included in each
compile unit that includes the `Instance` class. Eventually, if we get
rid of these intricate dependencies, we could remove the `Engine` class.
Pull Request: https://projects.blender.org/blender/blender/pulls/136001
92968c23fe