Fix gradients drawing garbage

* Our gradients have non-intuitive stops in the [0..255] range.
* They MUST have stops at 0 and 255, otherwise they draw random colors

Should fix most of the drawing glitches on various websites.
TestRunner: don't warn on known options

We should still make them actually work, however.
Fix PasteBoard issues:

* the text/plain chunks added to BClipboard aren't null terminated, so
we MUST take the length into account.
* The implicit constructor to WTF String takes latin-1 encoded strings.
We most likely have utf-8 input data, so use the method to build a
string from that.

Fixes #10250 and #10314.