OpenGL 3.x on macos

[j-w-c-b]

I was trying to build xu4 on my mac this weekend, and ran into some issues getting it to run. It uses some GLSL shaders to smoothly scale some of the 80s graphics, but they use modern GL features which the display returned by allegro5 doesn't support on macos.

From what I can tell the issues are

• passing a hint in the pixel format to get opengl 3
• updating the allegro default shaders to not use deprecated features
• injecting a #version xxx tag when compiling the shaders; macos is strict about that for the 3.2+ profiles---they must match the version used by the display.

I'll create a PR with my changes for this, but I started the weekend knowing nothing about shaders, so it might not be quite right, but is close.

[alemart]

Won't there be interoperability issues across platforms if a #version line is hard-coded into glsl_attach_shader_source() in src/opengl/ogl_shader.c as in #1444 ? Wouldn't it be preferable if the application sorted this out instead of Allegro?

https://github.com/liballeg/allegro5/blob/20df29dd32008ed86479644069fe95b01a83a7b3/src/opengl/ogl_shader.c#L127-L142