[omx] Restrict the number of outstanding jpeg calls to gpu

Allowing more than one outstanding call to gpu for texture encode/decode can be beneficial as processing
can be overlapped with vchiq message transfer.

Experimentally, 3 outstanding operations is optimal.

Allowing more that this ties up memory on GPU without any performance benefit, so avoid submitting more jobs
CGUIDialogContextMenu: remove "Add source" context menu entry
Merge pull request #5135 from Montellese/remove_add_source_context

CGUIDialogContextMenu: remove "Add source" context menu entry
Merge pull request #5091 from popcornmix/limitgpu

[omx] Restrict the number of outstanding jpeg calls to gpu
[videodb] adds UpdateLinkTable/UpdateActorLinkTable to video database
[videodb] adds UpdateDetailsForMovie() and calls it from JSON-RPC to more efficiently update movies.

Link tables are only updated if they need to be.
[videodb] only update art table if needed