Support writing to tape

[ColinPitrat]

At the moment, Caprice only supports loading from a tape, not writing to it.

Writing to the tape is done by writing to PPI port C, bit 5:
https://www.cpcwiki.eu/index.php/8255#PPI_Port_C

Currently, Caprice supports bit 4 (Cassette motor control) which modifies CPC.tape_motor in src/cap32.cpp. But it ignores bit 5.

Some information on how this works can be found here: https://www.cpcwiki.eu/index.php/Cassette_data_information

There are 2 main formats for tapes:

• CDT (https://cpctech.cpcwiki.de/docs/cdt.html - https://www.cpcwiki.eu/index.php/Format:CDT_tape_image_file_format)
• VOC - which are actually just a sound file (https://en.wikipedia.org/wiki/Creative_Voice_file)

One could also find WAV or AIV files but Caprice doesn't support them for now.

It should be fine to only support CDT for writing as external tools exist to convert from CDT to VOC or WAV (for example https://cpctech.cpcwiki.de/docs/cdtconv.html).

[ColinPitrat]

We want to store what's written on the tape in pbTapeImage.

The format of pbTapeImage is:

• 0x20 for a pause block, followed by its length in milliseconds (2 bytes)
• 0x15 for a direct recording block (which will use 1 bit per sample), followed by:
  • number of T states per sample (2 bytes)
  • pause after block (2 bytes)
  • bits used in last sample (1 byte)
  • data length in bytes (4 bytes)

This format is directly the one of a CDT file, so once written to pbTapeImage, a cdt_save method would simply need to write it in a CDT file (with an appropriate header). This wouldn't necessarily be the best encoding, not making use of most of the CDT features, but it should work.

Similarly for a VOC file, we could get away with writing only blocks of types 0 to 3.