Allows different implementations for different platforms (the JS
build relies on browser APIs to stream disk images over the network).
Setting aside the JS build, this also reduces some code duplication.
Result of running IWYU (https://include-what-you-use.org/) and
applying most of the suggestions about unncessary includes and
forward declarations.
Was motivated by observing that <thread> was being included in
ppcopcodes.cpp even though it was unused (found while researching
the use of threads), but seems generally good to help with build
times and correctness.
The disk cache is unchanged. data_ptr continues to be only used for the user data sector area for each block. The other sector areas (synch, header, etc.) are filled in while reading.
has_data and get_data exist as a way to bypass data_ptr for parts of the transfer outside the user data sector area of each block. The default behaviour is defined in atabasedevice and is overridden by atapicdrom for the Read CD command. atapicdrom has a flag doing_sector_areas to control the behavior of the get_data method. When the flag is true, the sector_areas, current_block, and current_block_byte are used for selecting the correct data from one of the sector areas. The Read CD command initializes those variables. xfer_cnt remains the total number of bytes to be transferred and is now not necessarily the same as the number of disk image blocks read into the disk cache.
lba_to_msf is used to fill in the header. The values was not verified using a real CD.
Mac OS X just cares about the Mode in the header. For now, only the synch and header and user data areas are filled in. The other areas read as all zeros.
In Read TOC format 2, Mac OS X passes zero for Session Number. I believe Read TOC is supposed to return the first session starting from that number so it should return info for Session 1 as it would if Mac OS X passed 1 for the Session Number.
