[apg_bmp] RLE flag for apg_bmp_write

[capnramses]

• e.g. in case you want to use fast BMP output to dump a screenshot without much of a stutter in your app.
• and don't wan't a folder of massive uncompressed true-colour images.
• note that this would only support 24-bit RGB indexed images, so more code may be required there than for RGBA 32-bit true-colours.
• the palette would have to be created using a table of colour distances, which might not be so fast.
• if an existing 256-colour palette was supplied as a field, then this would speed up the output using colour-distances.
• therefore 2 functions could be created:
  1. create_palette( from image )
  2. write_bmp( using palette )
• this might be more expedient to use in external tools to compress existing images or apply a palette to a folder of images