Provide minimal cross-platform GUI library

[jserv]

Fenster offers an extremely minimalistic and specific approach to displaying a cross-platform 2D canvas, characterized by its ease of use — simply include its header and begin making API calls. Integrating Fenster with rv32emu's SDL-based system calls could significantly enhance usability and simplify any future porting efforts, making it an ideal choice for streamlined, cross-platform graphical implementation.

[alanjian85]

I added the support of SDL-oriented system calls to fenster (See fenster-rv32emu) and also issued pull requests on every repositories related to the changes I made in rv32emu. Note that the port of fenster_audio.h is still under development, and some functions in fenster.h such as fenster_sleep() are not optimal because rv32emu lack of a sleep system call and it doesn't seem there is a way to estimate running time under emulation precisely.

[alanjian85]

The ported fenster library is still under my personal account. To provide a convenient access point for others to use, it is better to fork and maintain it under this organization. Besides, the code in fenster-rv32emu should be reviewed before forking it.