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st/egl: overload NATIVE_PARAM_PREMULTIPLIED_ALPHA
EGL does not export this capability of a display server. But wayland
makes use of EGL_VG_ALPHA_FORMAT to achieve it.
So, when the native display returns true for the parameter, st/egl will
set EGL_VG_ALPHA_FORMAT_PRE_BIT for all EGLConfig's with non-zero
EGL_ALPHA_SIZE. EGL_VG_ALPHA_FORMAT attribute of a surface will affect
how the surface is presented.
Because st/vega does not support EGL_VG_ALPHA_FORMAT_PRE_BIT,
EGL_OPENVG_BIT will be cleared.