Clear all EGL errors when entering EGL funcs.

This changes the clearError function in the EGL wrapper layer to simply
call eglGetError().  That should clear any pending errors from all the
underlying EGL implementations, which is needed to correctly report the
error for the most recently called EGL function.

Change-Id: Iad19c69f0c5305e873f3c2f96d353280d31f7b61