frontend: fix leaving FE canvases in bad state

[zavitax]

Description

This PR modifies the canvases vector to be
represented as a map of obs_canvas_t* to a
shared pointer to OBS::Canvas.

This will preserve both the OBS::Canvas manager
instance, and pointer to obs_canvas_t* for
reference by obs_frontend_canvas_remove(),
ensuring that the right objects will be used
when called upon.

Motivation and Context

The obs_frontend_remove_canvas is called, the
destructor of OBS::Canvas calls
obs_canvas_release() on an object which is
both allocated on the heap and used as reference.

This causes the canvases collection on the main
window to become corrupt, and crash the next time
you try to add another frontend canvas.

This effectively prevents the canvases collection
from being managed properly by plug-ins.

How Has This Been Tested?

Ran obs_frontend_add_canvas() followed by obs_frontend_remove_canvas() followed by obs_frontend_add_canvas() before and after the change.

Before change: crash
After change: smooth operation

Types of changes

• Bug fix (non-breaking change which fixes an issue)

Checklist:

• My code has been run through clang-format.
• I have read the contributing document.
• My code is not on the master branch.
• The code has been tested.
• All commit messages are properly formatted and commits squashed where appropriate.
• I have included updates to all appropriate documentation.