Adopt MutableDisposable for most usages of pattern <var>?.dispose(); <var> = undefined

[Tyriar]

This helps us from making mistakes by registering the clearing of an optional disposable when it's created, instead of having to create and dispose in two separate spots.

This also fixes the following issues:

• Texture atlases were getting disposed when canvas and webgl renderers were being disposed which could cause issues when they're shared. The disposal of the texture atlas is the duty of CharAtlasCache.removeTerminalFromCache
• The following weren't being disposed of correctly before:
  • Terminal._accessibilityManager
  • WebglRenderer._rectangleRenderer
  • WebglRenderer._glyphRenderer