add support for copy-screen-to-bitmap

[stevesims]

adds support for Acorn GXR style “copy screen to bitmap” command sequence

this is supported on both VDU 23,27,1,n,0,0,0 to define bitmap n from the last two graphics cursor positions, using an 8-bit bitmap ID and also VDU 23,27,&21,n;0; using a 16-bit bitmap/buffer ID

these commands are compatible with the existing VDU 23,27,1 and VDU 23,27,&21 commands - the system can tell the difference as these variants require a zero “height” value

if the buffer specified already exists then it will be deleted/cleared before the new bitmap is created from the screen. the resultant bitmap data will be in vdp-gl’s Bitmap::Native format

the original Acorn command documentation specifies that a few more zeros should be sent. you can safely send additional zeros, as per Acorn’s documentation, as they will just be ignored.

it should be noted that VDU 23,27,&21 when pointing to an existing buffer to set it to be a bitmap also usually expects a format byte, but this isn’t required/supported/needed for copying screen to bitmap, i.e. this command variant requires one less byte to be sent

requires an updated/custom vdp-gl, and can thus currently only be built using PlatformIO