Default to libGLESv2 for EGL surface buffer creation.

Default to libGLESv2 for EGL surface buffer creation.

If both libGLES_CM and libGLESv2 are available, we were using the
former for creating the buffers of an EGL surface. The choice is
arbitrary, but it causes confusing issues when an interface changes
and one of the libraries wasn't rebuilt. This is less likely to occur
for libGLESv2.

Change-Id: If0c05d50a141c3782866892694d238332d8211db
Reviewed-on: https://swiftshader-review.googlesource.com/13388
Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>