gdi32: Keep track of the number of unique fonts that are created and return this in the second DWORD of the GdiRealizationInfo structure.

This is mostly for correctness, in practice we should always be able
to avoid using CTXUSAGE_BLIT for offscreen targets when FBO ORM is
used.

Fixes some GL errors due to calling glDrawBuffer(GL_BACK) when an FBO
is still active.

As pointed out by Alexander Dorofeyev.

CTXUSAGE_CLEAR will apply the FBO state in a later patch.