Fix misuse of read and skipBytes

[riccardobl]

Fixes #2555 across the entire engine.

This PR fixes the implementation of LittleEndien.readFully() and introduces two new utility methods in ByteUtils:

• readFully
• skipFully

Both ensure that the requested number of bytes are reliably read or skipped, even if the underlying stream does not return the full amount in a single call.

The changes are not yet fully tested, but they are part of the codebase I am actively working with. Additional testing and verification from others would be greatly appreciated.

[ScottPeterJohnson]

This patch may (still) actually cause an issue with GLTF loader. Essentially requiring the "full skip" and erroring on EOF can cause failure. From an asset I'm working with:

int end = count * stride + byteOffset; //1839200, when the actual byte array backing it is only 1839188 long
        stream.skipBytes(byteOffset);
        while (index < end) {
            //Index is 1839136, reads the last value
            for (int i = 0; i < numComponents; i++) {
                buffer.put(readAsFloat(stream, format));
            }
            if (dataLength < stride) {
                //Tries to skip 52 bytes; but 1839136+52=1839200 > 1839188
                stream.skipBytes(stride - dataLength);
                //Throws EOFException here
            }
            index += stride; //Would set index to 1839200 and skip loop
        }

(I'd add the asset as example since it messes up so much, but it's Unreal licensed.)