1.5a Update - Small Update

Yeah, not a huge update. Just fixing a few small things as of right now. I promise to eventually come back with a much bigger update.

List of changes:

• Fixed bug where getting rid of a set custom texture map instance by setting the array count low enough on a PaperPolygon2D didn't clear it from the associated spatial material.
• Hardcoded the limit on the amount of custom texture maps per PaperPolygon2D to prevent potential game dev mistakes. (I'm certain that no sane person is going to use any more than 32 textures per material)
• Changed how PaperPolygon2D safety checks the index parameter for certain custom texture map functions to allow for negative input.
• `paper_complex_shader` and `paper_complex_with_custom_textures_shader` tweaks:
• Added check to see if the polygon group's `child_order_changed` signal is actually connected before trying to disconnect it.
• Fixed bug where PaperSkeleton wouldn't clear a signal of a different polygon group if it was set to a different one.
• Fixed bug where `_clear_session_data()` cleared the canvas z-index instead of the paper z-index. Sorry if this bug caused anyone any frustration.
• Left in a single emissive texture for the example rig shown in my new showcase video, 'cuz I couldn't think of really any reason to remove it.
  • Also, yeah, I made a new video properly showcasing the addon! If you use the addon, there's a decent chance you're already aware of most of the addon's features, but still, I though it was worth mentioning.

As always, please feel free to report any bugs if you come across any, even minor ones.