Fix/linux desktop capturer x11 abi

[Isacco-B]

Problem

On Linux with WebRTC m140 (branch 7339), constructing DesktopAndCursorComposer
crashes with SIGSEGV. The root cause is an ABI mismatch in DesktopCaptureOptions:

The prebuilt WebRTC library is compiled with use_x11=true, which adds
X11-specific fields to DesktopCaptureOptions (conditional on WEBRTC_USE_X11).
The JNI CMake build already links against X11 Xfixes Xrandr Xcomposite but did
not define WEBRTC_USE_X11 as a preprocessor symbol. This caused the struct
layout seen by JNI code to differ from the one in the library → reading out of
bounds when passing options to DesktopAndCursorComposer → SIGSEGV.

This affects all Linux architectures (x86-64, arm, arm64) and all distributions.
It was introduced as a regression with the m140 update.

Fix

1. Root cause (CMakeLists.txt): add WEBRTC_USE_X11 to Linux compile
   definitions so the JNI code and the WebRTC library agree on the struct layout.

2. JNI safety (DesktopCapturer.cpp, JNI_ScreenCapturer.cpp,
   JNI_WindowCapturer.cpp): replace the throw in the constructor (which would
   cross the JNI boundary as undefined behavior) with a log + early return, and
   add null-guards on all methods. The JNI entry points now catch any remaining
   C++ exceptions and convert them to Java exceptions.

Testing

Verified on Ubuntu 24.04 (X11 session), linux_x86-64.
Screen and window capture work correctly.