Change trigger_multi_frame_capture() to take &mut self

[ebkalderon]

Currently, all of the following capture-related API methods take &mut self:

1. trigger_capture()
2. start_frame_capture()
3. end_frame_capture()

This is because the RenderDoc API used for capturing frames is designed with inherently serial operations (not thread-safe) and need to statically indicate to the compiler and end user alike that unsynchronized internal mutation is occurring. As such, trigger_multi_frame_capture() should take &mut self as well.

This is technically a breaking API change, but it is critical to fix this in order to prevent unsound behavior in future versions. It should be documented in the CHANGELOG.md so users looking to upgrade versions are aware of the change.