Blender Git Loki

Cleanup: Use `_` suffix for non-public members in Compositor

To follow the style guide.

Cleanup: Convert camelCase naming to snake_case in Compositor

To follow the style guide.

Cleanup: Remove `this->` for `m_` prefixed members in Compositor

The prefix is already explicit.

Cleanup: Remove unused includes in Compositor

And move unneeded includes in frequently used headers
to source files.

Slightly reduces compilation time.