Add gles2 tag to support using ANGLE (EGL+ OpenGLES2) on windows

[zwang]

while still keep the Desktop OpenGL option default for windows.

With this change, the go-gl/gl binding can use EGL+OpenGL ES2 as an option by passing egl and gles2 tags. (using ANGLE project)

Without these tags, it can still use the default desktop OpenGL as before

Should help on: #51

[errcw]

I'm curious why the "2" in the gles2 tag? As I understand it, ANGLE supports both GLES 2.0 and 3.x?

[zwang]

Yeah, that's true. gles can works too.

I think I was in the mindset that I was fixing for OpenGL ES 2 and also the library from angle is libGLESv2.dll. Also I saw this tag is already being used in go-gl/glfw repo for linux,gles2

From another perspective, gles2 also can mean OpenGL ES 2 and above, which differentiates from OpenGL ES 1. Nobody should be using OpenGL ES 1 at this moment. But using gles might indicate it support OpenGL ES 1 too.

I am OK to change. But I prefer to keep this gles2 to be consistent with other places in GLFW.

Maybe should add gles3 too if we want to make it cover OpenGL ES 3.x

[errcw]

Thanks for thinking this through! I agree consistency is a good argument.