SDL build improvements

[ryandesign]

Here are several improvements to the scons/SDL/OpenGL build, especially for macOS.

Update GitHub workflow: also build SDL UI on macOS

Import PATH into scons environment

Fixes

/bin/sh: sdl2-config: command not found

when sdl2-config is not in a standard system bin directory (e.g. MacPorts).

Use the right include path for SDL.h

Fixes

main.cpp:22:10: fatal error: 'SDL2/SDL.h' file not found

when SDL2 is not in a standard system include directory (e.g. MacPorts).

Link with Accelerate framework on macOS

Fixes

Undefined symbols for architecture x86_64:
  "_vDSP_dotpr_s1_15", referenced from:
      Outputs::Speaker::LowpassBase<Outputs::Speaker::PushLowpass<true>, true>::resample_input_buffer(int) in Audio.o
      Outputs::Speaker::LowpassBase<Outputs::Speaker::PullLowpass<GI::AY38910::AY38910<true> >, true>::resample_input_buffer(int) in AmstradCPC.o
      Outputs::Speaker::LowpassBase<Outputs::Speaker::PullLowpass<Audio::Toggle>, false>::resample_input_buffer(int) in AppleII.o
      Outputs::Speaker::LowpassBase<Outputs::Speaker::PullLowpass<Outputs::Speaker::CompoundSource<Apple::IIgs::Sound::GLU, Audio::Toggle> >, false>::resample_input_buffer(int) in AppleIIgs.o
      Outputs::Speaker::LowpassBase<Outputs::Speaker::PullLowpass<Apple::Macintosh::Audio>, false>::process(unsigned long) in Macintosh.o
      Outputs::Speaker::LowpassBase<Outputs::Speaker::PullLowpass<Apple::Macintosh::Audio>, false>::update_filter_coefficients(Outputs::Speaker::LowpassBase<Outputs::Speaker::PullLowpass<Apple::Macintosh::Audio>, false>::FilterParameters const&) in Macintosh.o
      Outputs::Speaker::LowpassBase<Outputs::Speaker::PullLowpass<Atari2600::TIASound>, false>::process(unsigned long) in Atari2600.o
      ...
ld: symbol(s) not found for architecture x86_64

Silence macOS OpenGL deprecation warnings

Removes warnings like this:

main.cpp:900:2: warning: 'glGetIntegerv' is deprecated: first deprecated in macOS 10.14 - OpenGL API deprecated. (Define GL_SILENCE_DEPRECATION to silence these warnings) [-Wdeprecated-declarations]

Link with OpenGL framework on macOS

Fixes

ld: library not found for -lGL

Improve Macintosh video & Apple II colors on macOS

Hacks in AppleII/Video.cpp, AppleII/Video.hpp, and Macintosh/Video.cpp assume that building on macOS means building for Metal unless IGNORE_APPLE is defined. By defining this in the macOS SDL build, Macintosh video is now sized and positioned correctly and Apple II colors are now just as wrong as they are on other OpenGL builds instead of being wrong in a unique way.

See #872

Fix OpenGL Apple II colors

Adjust phase by 90 degress.

Closes #872

Change phase from int to uint8_t

output_colour_burst expects a uint8_t so may as well make that clear.

[TomHarte]

In my mild defence, I've never actually intended to support SDL on the Mac — it's purely there for development convenience. This is still an improvement though.

[ryandesign]

Oh sure. But hey, now it works!