Fix BufferedGraphicsMode::clear() so that it fills all pixels

[zyla]

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PR description

Currently clear() fills the buffer with 0x01 bytes, which represent
7 dark pixels and 1 bright pixel. We want all pixels bright. This PR fixes it by filling buffer with 0xff.

[jamwaffles]

Well spotted. Thanks!

[jamwaffles]

This is released in 0.8.4. cargo update should pull the change in.

[zyla]

Thanks, that was very fast!