EvasGL: Fixed a few minor bugs.

EvasGL: Fixed a few minor bugs.
- Added (w,h) <=0 dimension check for evas_gl_surface_create()
- Changed evas_gl_make_current to return error when either
  surface or context is NULL.  Semantically, this was allowed
  before but it was changed to reflect eglMakeCurrent behavior.
- evas_gl_make_current - detached any previously attached
  buffers before attaching new ones to an FBO during a make_current.
- Used dynamic memory for extension string allocation for safety.

git-svn-id: svn+ssh://svn.enlightenment.org/var/svn/e/trunk/evas@72926 7cbeb6ba-43b4-40fd-8cce-4c39aea84d33