We don't need intrinsic scale factors for Gfx::Bitmap in Ladybird,
as everything flows through the CSS / device pixel ratio mechanism.
This patch also removes various unused functions instead of adapting
them to the change.
a396bb0 removed the palette field but did not update the allocation size
in `Bitmap::serialize_to_byte_buffer()`. This led to a few crashes (I
noticed this from a drag/drop crash in the file manager).
Fixes#21434
Since the color interpolation requires two pixels in the horizontal and
vertical direction to work, 1 pixel wide or high bitmaps would cause a
crash when scaling. Fix this by clamping the index into the valid range.
Fixes#16047.
