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add support for copy-screen-to-bitmap#153

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stevesims wants to merge 2 commits intobreakintoprogram:mainfrom
stevesims:upstream-screen-to-bitmap
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add support for copy-screen-to-bitmap#153
stevesims wants to merge 2 commits intobreakintoprogram:mainfrom
stevesims:upstream-screen-to-bitmap

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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

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
fixes issue with capturing bitmap to buffer zero
(the `createBuffer` call was preventing creation of buffer with ID 0, as it can’t be used as a place to redirect output)
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