GLSL version #120 is no longer supported on the Mac

[ThrudTheBarbarian]

I got SDL_GPU working by changing the code to change the version from #120 to #150, and rewriting the shaders to be 150-compliant:

• shader inputs are 'in' not 'attribute'
• shader outputs are 'out' not 'varying'
• fragment shader outputs go to a user-defined 'out vec4 fragColor' instead of a predefined gl_FragColor
• fragment shader 'texture2D' calls are just 'texture' calls

This because on OSX Monterey at least, I see in the 'GPU_Renderer' struct:

min_shader_version int 150 max_shader_version int 410

... so it fails to load the #version 120 code.