Previously, Godot only imported additional UV coordinates (3 to 8) into CUSTOM0, CUSTOM1, CUSTOM2. This commit makes the following changes:
1. Allow import of _CUSTOM0, _CUSTOM1, _CUSTOM2, _CUSTOM3, defined as 4 floating pointer numbers each. These are given priority over UV coordinates as custom attributes.
2. Increase the limit of UV imports from 3 pairs (UV3-UV8) to 4 pairs, so _CUSTOM3 can be supported via UV9 & UV10, if desired. These are imported only if a corresponding _CUSTOMn attribute doesn't exist. Note that Blender only supports 8 UV sets in total; therefore Blender users must use _CUSTOM3, defined as "Color" (so it's 4 floats) to import into Godot's CUSTOM3.
3. Increase the limit of UV exports from 3 pairs to 4 pairs, so that exported glTFs retain all the custom vertex attribute values, and CUSTOM3 will retain its values when the file is reimported. The exporter continues to use UV sets to store the custom attributes, with UV9 & UV10 used for CUSTOM3.
In essence, this commit allows one to define CUSTOM3, while also improving the import options for the other CUSTOM shader built-ins.
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