LibWeb/WebGL: Replace manual pointer math with Span

This deduplicates a lot of sensitive pointer math by using Span, which
performs this math for us with more safety checks and with type
information.

This also allows us to use the correctly typed Span for typed arrays,
which automatically fixes srcOffset to now offset by the number of
elements instead of bytes. This goes for srcLengthOverride too.

Fixes the Rive animations on Shopify's homepage not appearing.
Fixes some Unity applications such as ArcViewer having missing
graphics.

Author: Lubrsi