Feature: Custom texture size

[Pituivan]

What I changed:

Issue #22 asked for support for packing non-square textures, so I added a toggle Custom texture size before the Resolution field which, when checked, replaces the Resolution field with a couple of options to set a custom texture width and height. As requested in the mentioned issue, the default custom size is the size of the first created input (if any) unless it's created before checking the Custom texture size toggle.

Changes in the UI:

The mentioned toggle, located right on top of the Resolution field.

All the new options to set a custom texture size, which appear when the toggle is checked. Note that the Use size from input button only appears when the user created at least one input.

When clicking the Use size from input button, you get to choose from which of the inputs you added you want to import the texture size.

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Closes #22