Unify all size_t arguments to use UIntPtr

[DemoXinMC]

Addresses #212

• Updated to use a SIZE_T "typedef" where CSFML is expecting size_t

• using SIZE_T = UIntPtr; is at the beginning of each file that will P/Invoke functions with size_t signatures

• SIZE_T is currently typedef'd to UIntPtr but this can be changed when nuint becomes available to SFML.NET (.NET 5+)

• Since all changes are within protected/private code, this should not represent a breaking change for end-users

• Implemented Font(byte[]) properly using sfFont_createFromMemory() instead of creating a new MemoryStream