DRTVWR-418: Unify control flow through LLAppViewer across platforms.

The LLApp API used to consist of init(), mainLoop(), cleanup() methods. This
makes sense -- but on Mac that structure was being subverted. The method
called mainLoop() was in fact being called once per frame. There was
initialization code in the method, which (on Mac) needed to be skipped with an
already-initialized bool. There was a 'while' loop which (on Mac) needed to be
turned into an 'if' instead so the method would return after every frame.

Rename LLApp::mainLoop() to frame(). Propagate through subclasses LLAppViewer
and LLCrashLogger. Document the fact that frame() returns true to mean "done."
(This was always the case, but had to be inferred from the code.)

Rename the Mac Objective-C function mainLoop to oneFrame. Rename the C++ free
function it calls from runMainLoop() to pumpMainLoop(). Add comments to
llappdelegate-objc.mm explaining (inferred) control flow.

Change the Linux viewer main() and the Windows viewer WINMAIN() from a single
LLAppViewer::mainLoop() call to repeatedly call frame() until it returns true.

Move initialization code from the top of LLAppViewer::frame() to the init()
method, where it more properly belongs. Remove corresponding
mMainLoopInitialized flag (and all references) from LLAppViewer.

Remove 'while (! LLApp::isExiting())' (or on Mac, 'if (! LLApp::isExiting())')
from LLAppViewer::frame() -- thus unindenting the whole body of the 'while'
and causing many lines of apparent change. (Apologies to reviewers.)

There are four LLApp states: APP_STATUS_RUNNING, APP_STATUS_QUITTING,
APP_STATUS_STOPPED and APP_STATUS_ERROR. Change LLAppViewer::frame() return
value from (isExiting()) (QUITTING or ERROR) to (! isRunning()). I do not know
under what circumstances the state might transition to STOPPED during a
frame() call, but I'm quite sure that if it does, we don't want to call
frame() again. We only want a subsequent call if the state is RUNNING.

Also rename mainLoop() method in LLCrashLogger subclasses
LLCrashLoggerWindows, LLCrashLoggerMac, LLCrashLoggerLinux. Of course it's
completely up to the frame() method whether to yield control; none of those in
fact do. Honor protocol by returning true (frame() is done), even though each
one's main() caller ignores the return value.

In fact LLCrashLoggerWindows::mainLoop() wasn't using the return protocol
correctly anyway, returning wParam or 0 or 1 -- possibly because the return
protocol was never explicitly documented. It should always return true: "I'm
done, don't call me again."

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