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The GraphicsV spec says that the overlay-related calls should only be
called from the foreground (or a similar non-interrupt context). For
BCMVideo's case, if they're called from IRQ handlers then things will
likely break, because the module won't be able to yield properly to wait
for the Dispmanx service connection to be free, or to wait for replies
from the GPU within VCHIQSend.

Although it might be possible to relax the spec to allow some calls to
be made from IRQ handlers (with BCMVideo queueing the changes to apply
them at a later date, as with some pointer updates), for now let's just
go with the approach of having the module check the calling context and
ignore the call if it's being made from the background.

This fixes the problem reported on the forums with unwanted overlays
being left on screen by the (experimental) threaded ADFFS modules. Now
the overlays simply won't be displayed at all, since the calls to
display them were being made from within the VSync IRQ handler.

https://www.riscosopen.org/forum/forums/4/topics/19002#posts-146116