Problem: A borked process sending a Gfx::Bitmap (inside BitmapSequence)
or a Gfx::ShareableBitmap over IPC could set BitmapFormat::Invalid as
the format field. The receiving process then aborted while decoding the
message — an IPC-reachable crash.
Cause: The helper that both decoders use for validating the format read
off the wire accepts BitmapFormat::Invalid. The decoders go on to build
a bitmap with that format. But that triggers an assert — because
minimum_pitch only knows the four real pixel formats.
Fix: Drop BitmapFormat::Invalid from is_valid_bitmap_format. It’s an
indicator of an absent/unknown format, never one a real bitmap can have.
And any real bitmap is never encoded with it. So, both BitmapSequence
and ShareableBitmap decode now return a clean decode error for it.
