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.
While Emscripten has an SDL compabtility layer, it assumes that the
code is executing in the main browser process (and thus has access to
them DOM). The Infinite Mac project runs emulators in a worker thread
(for better performance) and has a custom API for the display, sound,
input, etc. Similarly, it does not need the cross-platform sound support
from cubeb, there there is a sound API as well.
This commit makes SDL (*_sdl.cpp) and cubeb-based (*_cubeb.cpp) code be
skipped when targeting Emscripten, and instead *_js.cpp files are used
instead (this is the cross-platform convention used by Chromium[^1], and
could be extended for other targets).
For hostevents.cpp and soundserver.cpp the entire file was replaced,
whereas for videoctrl.cpp there was enough shared logic that it was
kept, and the platform-specific bits were moved behind a Display class
that can have per-platform implementations. For cases where we need
additional private fields in the platform-specific classes, we use
a PIMPL pattern.
The *_js.cpp files with implementations are not included in this
commit, since they are closely tied to the Infinite Mac project, and
will live in its fork of DingusPPC.
[^1]: https://www.chromium.org/developers/design-documents/conventions-and-patterns-for-multi-platform-development/
Use explicit cast when converting large integer types to smaller integer types when it is known that the most significant bytes are not required.
For pcidevice, check the ROM file size before casting to int. We'll allow expansion ROM sizes up to 4MB but usually they are 64K, sometimes 128K, rarely 256K.
for machinefactory, change the type to size_t so that it can correctly get the size of files that are larger than 4GB; it already checks the file size is 4MB before we need to cast to uint32_t.
For floppyimg, check the image size before casting to int. For raw images, only allow files up to 2MB. For DiskCopy42 images, it already checks the file size, so do the cast after that.
