Declare fragment shader outputs, improving compatibility (macOS)

[fabjan]

Problem

The build went fine without warnings or errors on my Mac, but the shaders cannot be compiled:

❯ ./ded     
GL version 3.3
WARNING: GLEW_ARB_debug_output is not availableERROR: could not compile GL_FRAGMENT_SHADER
ERROR: 0:6: Use of undeclared identifier 'gl_FragColor'

ERROR: failed to compile `./shaders/simple_color.frag` shader file

Is the program written for a different GL version than what macOS 11.7.2 provides perhaps (see caveats)?

Solution

Update each of the shaders/simple_*.frag programs to stop using gl_FragColor, instead declare out vec4 fragColor; and use that.

Fixes #65.

Compare for this fix in dlangui:

buggins/dlangui@6cfe98a#diff-0671d7c48bc6543434a3b3c9b6671be5b41902e679cfa0520fa7f0c780d670e4L231

Caveats

I don't write many graphical programs. Perhaps macOS is always a pain and you don't want to support it. Given that build.sh contains a uname check for "Darwin" though, I'm giving it a shot.

I used the following versions of the dependencies (latest in Homebrew):

• sdl2 2.26.3
• freetype 2.13.0
• glew 2.2.0_1