EGL: use eglGetPlatformDisplay if available

[thesourcehim]

To properly use prime render offload eglGetPlatformDisplay must be used instead of eglGetDisplay. It's available in EGL 1.5, so use it if available, fallback to eglGetDisplay if not. Helps #864.

[rogerman]

We can assume that the EGL v1.5 headers will be available at compile time, so no need for #ifdef here. (However, we are still requiring EGL v1.4 at runtime.)

Also, your name of "attr" isn't descriptive enough, since there are other attributes in that particular function. Please rename "attr" to "displayAttr", thanks!

[thesourcehim]

I guess I just try all 3 calls to obtain display handle, I don't see a way to obtain runtime EGL version before calling eglInitialize.