We need to use a resource guard for the FBO in case there is no
current context when the glyph cache is deleted.
This reverts commit b3264e2cb6 which
was implemented as a band-aid for this crash.
Change-Id: I5b3a09a3998da38836ea851cd0978d3ddadcd2cc
Reviewed-by: Samuel Rødal <samuel.rodal@digia.com>