Fix: BLOAD was silently misreading bitfields for BMPs with the undocumented BITMAPV3HEADER format (56-byte) headers.
It was treating them the same as the older 24-byte header, which placed the bitfield values after the header (where the colour table is), so was looking for them after the header.
Disallow unknown BMP header sizes, to prevent them being misread.
Allowed sizes are:
    12: OS/2 V1 (BITMAPCOREHEADER)
    40:  BITMAPINFOHEADER
    56:  BITMAPV3HEADER (undocumented)
    108: BITMAPV4HEADER
    124: BITMAPV5HEADER
Disallow commas/newlines after SPC/TAB
They aren't that useful, and were broken - acting as if a semicolon followed.
Now only semicolons are allowed, but that could be reexamined in future releases.
There is currently no implementation support for them though, so the code would have to be complicated a little.
Forgot this rename in last commit
Fix LINE bug added in recent commit.
Clipping case fixed so y1 and y2 are updated, otherwise SET_DIRTY could be passed out-of-bounds values.
As it was, y2 could be one too high/low, and y1 was never clipped at all.