Hide a bunch of debug logging statements in #ifdef SHOW_DEBUG to prevent spinning a mutex in hot paths.

requestExit() doesn't close CEF, command sets CEF into closing state which will be processed on update()

This reverts commit f8ccb750

This reverts commit f8ccb750.

LLThread::currentID() used to return a U32, a distinct unsigned value
incremented by explicitly constructing LLThread or by calling LLThread::
registerThreadID() early in a thread launched by other means. The latter
imposed an unobvious requirement on new code based on std::thread. Using
std::thread::id instead delegates to the compiler/library the problem of
distinguishing threads launched by any means.

Change lots of explicit U32 declarations. Introduce LLThread::id_t typedef to
avoid having to run around fixing uses again if we later revisit this decision.

LLMutex, which stores an LLThread::id_t, wants a distinguished value meaning
NO_THREAD, and had an enum with that name. But as std::thread::id promises
that the default-constructed value is distinct from every valid value,
NO_THREAD becomes unnecessary and goes away.

Because LLMutex now stores LLThread::id_t instead of U32, make llmutex.h
#include "llthread.h" instead of the other way around. This makes LLMutex an
incomplete type within llthread.h, so move LLThread::lockData() and
unlockData() to the .cpp file. Similarly, remove llrefcount.h's #include
"llmutex.h" to break circularity; instead forward-declare LLMutex.

It turns out that a number of source files assumed that #include "llthread.h"
would get the definition for LLMutex. Sprinkle #include "llmutex.h" as needed.

In the SAFE_SSL code in llcorehttp/httpcommon.cpp, there's an ssl_thread_id()
callback that returns an unsigned long to the SSL library. When LLThread::
currentID() was U32, we could simply return that. But std::thread::id is very
deliberately opaque, and can't be reinterpret_cast to unsigned long.
Fortunately it can be hashed because std::hash is specialized with that type.

Brought back the changes from @andreyk for improved mouse scroll wheel performance (previously 8e228364f324)

Pull in CEF 75.0.1+gb5e74dd+chromium-75.0.3770.80 and Dullahan 1.2.x along with some minor compatibility changes

Changes from @andreyk for improved mouse scroll wheel performance and pull in new Dullahan with CEF 74.1.19_gb62bacf_chromium-74.0.3729.157

SL-10811 - [Render][OSX] Google’s search results page is not fully displayed in the Media Browser if HiDPI is enabled.
SL-10813 - [Render][OSX] HiDPI browser has black bars on the sides.
Fixed.

MAINT-8325 Fixed The Save Local dialog disconnects the viewer if you do not choose the save directory files within the first minute

libatmosphere integrated in indra/llrender/llatmosphere.cpp

Still working on runtime shaders to use libatmosphere precomputed atmospherics textures

Fixes for: MAINT-8244 Text in CEF cannot be selected with the mouse. MAINT-8245 Expose CEF log file and logging severity to viewer and MAINT-8246 Expose the CEF remote debugging system to the viewer

Fixes for 'MAINT-8196 Remove LLPluginCookieStore from the viewer' and 'MAINT-8194 Remove per-frame calls to updateJavascriptObject()'

Pull in Dullahan 1.1.930 with improved fix for maint-8095 (target='_blank' links) as well as an improvement for maint-8100 (no error message for invalid hostname / url)