Update to use libsdl2

[craigthomas]

This PR updates the emulator to use SDL 2.0 instead of the deprecated SDL 1.2 interface. Most unit tests remain unaffected. Github Actions workflow updated to install correct libraries. This PR closes #51

[codecov]

Codecov Report

Attention: Patch coverage is 72.30769% with 18 lines in your changes missing coverage. Please review.

Project coverage is 91.68%. Comparing base (09f5b4c) to head (9cf056c).

Files with missing lines	Patch %	Lines
src/screen.c	72.13%	17 Missing ⚠️
src/cpu.c	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #52      +/-   ##
==========================================
- Coverage   92.08%   91.68%   -0.41%     
==========================================
  Files           9        9              
  Lines        2678     2681       +3     
  Branches      235      237       +2     
==========================================
- Hits         2466     2458       -8     
- Misses        207      218      +11     
  Partials        5        5              

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