Make OMTC work on OS/2

[dmik]

OMTC stands for off-main-thread compositing used in Firefox to improve performance and such. OMTC uses the same IPDL code (which abstracts cross-thread and cross-process communication in Mozilla to run various parts of functionality in separate threads or child processes) that is used by what they call e10s or Electrolysis. It seems that OMTC only involves cross-thread communication (as opposed to various other parts which use cross-process stuff, see #106 and other related tickets) so it deserves its own ticket.

OMTC is described in detail here: https://wiki.mozilla.org/Platform/GFX/OffMainThreadCompositing. Electrolysis is described here: https://wiki.mozilla.org/Electrolysis.

It's not clear ATM what we should do besides IPDL (in its cross-thread communication part) to make this OMTC approach work on OS/2 and if it's worth investing time in this at all.

[dmik]

As OMTC which turns out to be ON by default in ESR 45 and above just gives black window contents at the moment, we should disable it by default on OS/2.

[dmik]

In the above commit I disabled OMTC on OS/2 by default. We should roll it back when we make it work.