glcontext/egl: Prefer GLES2 over GL/GL3 by default
From a multimedia perspective GLES >= 2 has the big advantage of
supporting external textures (OES_EGL_image_external /
OES_EGL_image_external_essl3), allowing various YUV formats to be
imported directly by drivers.
It appears unlikely by now that the extension will ever be ported to GL with Vulkan becoming more popular, leaving GL without an "official" way to import YUV formats.
Further more, for Gst internal purposes it's likely that GLES2 works equally well if not better on most drivers these days, especially on embedded devices.
Thus switch the default for EGL context creation to GLES2. This won't
affect apps that create their own context, but gst-launch-1.0 etc.,
which are often used for testing so people don't have to pass
GST_GL_API=gles2.
Context:
cc @ndufresne @slomo