Screenshot Plugin: Re-added GL_BLEND enabling and disabling calls to fix the selection rectangle not being transparent anymore.
Make sure to use GL_BLEND for the non-GLES codepath only.
Those calls got lost in r3320 (GLES merge).
(LP: #1047788). Fixes: https://bugs.launchpad.net/bugs/1047788. Approved by Timo Jyrinki.