Remove unnecessary code in Texture2DConverter

[isHarryh]

Summary

This PR does some optimizations to the Texture2DConverter:

• Removes unnecessary paste call in the pad_image, improving performance by ~20%.
• Removes unused function assert_rgba.
• Removes unnecessary if not instanceof(xxx, bytes) check before converting xxx to bytes. In CPython, bytes(xxx) will return xxx itself if xxx is already a bytes.

This PR is a part of #318