Fix error 1280 in MacOS when querying GL_FRAMEBUFFER_SRGB_CAPABLE_EXT

[riccardobl]

The line getBoolean(GLExt.GL_FRAMEBUFFER_SRGB_CAPABLE_EXT) in GLRenderer throw an opengl error 1280 (invalid enum) when the GL debugger is enabled.
I am not sure why this is the case, however i believe it is safe to assume that every modern macos device is sRGB capable.

[riccardobl]

I reconsidered this PR, it just removes a warning, so it is better to exclude all the mac os platforms, worst case scenario: the devs won't receive a warning when using sRGB on unsupported deprecated platforms, not a big deal.

There isn't a way to get the generic os name (ie without the arch) from Platform in jme, so i've added that aswell, it will be useful to apply os specific workarounds across the engine.

[stephengold]

Another way to determine the OS is to invoke org.lwjgl.system.Platform. I'm unsure which is better in the long run.

[riccardobl]

We can't use org.lwjgl.system.Platform outside of the jme3-lwjgl3 modules

[stephengold]

Regarding "Platform.java", I have some style requests:

• Please provide javadoc for the new public class and the new public method.
• Please provide a space character after each new comma, before each new curly brace, and on both sides of each assignment operator.

[riccardobl]

done

[stephengold]

Ready for integration, I think.

[stephengold]

Thanks, Riccardo!