It also now caches at the higher level, so the platform-specific bits don't
change their interface much.
A little code hygiene work was applied to some of the platform bits on top of
this.
Reference Issue #10229.
Adds functions to query the system's realtime clock, convert time intervals to/from a calendar date and time in either UTC or the local time, and perform time related calculations.
An SDL_Time type (a time interval represented in nanoseconds), and SDL_DateTime struct (broken down calendar date and time) were added to facilitate this functionality.
Querying the system time results in a value expressed in nanoseconds since the Unix epoch (Jan 1, 1970) in UTC +0000. Conversions to and from the various platform epochs and units are performed when required.
Any direct handling of timezones and DST were intentionally avoided. The offset from UTC is provided when converting from UTC to a local time by calculating the difference between the original UTC and the resulting local time, but no other timezone or DST information is used.
The preferred date formatting and 12/24 hour time for the system locale can be retrieved via global preferences.
Helper functions for obtaining the day of week or day or year for calendar date, and getting the number of days in a month in a given year are provided for convenience. These are simple, but useful for performing various time related calculations.
An automated test for time conversion is included, as is a simple standalone test to display the current system date and time onscreen along with a calendar, the rendering of which demonstrates the use of the utility functions (press up/down to increment or decrement the current month, and keys 1-5 to change the date and time formats).
This removes the "FSops" naming scheme, which was meant to mirror the
"RWops" naming scheme, which was also recently removed from SDL3.
The build system defines (`SDL_FSOPS_POSIX`, etc) and the source code
filenames retain this, because there's already things using the word
"filesystem" that might overlap (for example, lots of things have a
unique "SDL_sysfilesystem.c", to query base dirs, etc, but almost
everything uses the POSIX "SDL_sysfsops.c" source code.
Fixes#9288.
