implement wait for update ioctl

[raisjn]

in mxcfb on rM1, using ioctl there is ability to do:

send_upate { update_marker: X };
wait_for_update { update_marker: X};

in rm2fb, there is no longer the "wait for update" ability due to two things:

1. there's only single way communication from client -> server
2. the framebuffer API has a command, "waitForLastUpdate()" but it does not support supplying a marker ID (i think). translating "send_update; wait_for_update ID" into "send_update; waitForLastUpdate()" requires knowing when to call waitForLastUpate() after drawing an update, but the wait_for_update can come at any arbitrary time and we process the incoming events in order.

this task is to give the ability for rM2 apps to wait for a previous draw to finish.

[raisjn]

some possible solutions:

• hardcode some amount like 200ms (see implement wait for update ioctl #44)
• add a function that guesses the draw time based on region size and waveform
• implement event system from server that client can subscribe to

[raisjn]

another solution proposed (by bokluk) is to use semaphores and would work something like:

when WAIT ioctl is issued on the client:

• the client places a special messages into the IPC queue. the message contains the ID of the semaphore to signal.
• then the client waits on that semaphore (with a timeout just in case).
• when the server gets to processing this message, it opens the indicated semaphore and signals that its ready

[raisjn]

first attempt in #45. this solves some weird behavior i was seeing in an app previously and i expect it should work in other places too