[Linux] fix: make textures thread-safe on linux

[Kingtous]

We've encountered some memory corruption when developing RustDesk, which is an open-source remote desktop built with Flutter, and it uses the texture API provided by Flutter to provide the remote screen to clients.

Both Windows and macOS work well, except for Linux. Sometimes random crashes will show and print different stack traces in these conditions:

• after registering a new texture
• preparing a RGBA data and markAvaliable

The runtime often complains double free or corruption.
One of the stack traces using lldb-10:

We checked the implementation of calling textures API on Linux several times, but have no luck.

We've noticed that on Linux, textures are organized by g_hash_table, which is not the thread-safe hashtable and we should ensure synchronization on different threads. Also, we noticed the textures map on Windows already has the synchronization (for example

engine/shell/platform/windows/flutter_windows_texture_registrar.cc

Lines 87 to 90 in 2c342a8

	{
	std::lock_guard<std::mutex> lock(map_mutex_);
	auto it = textures_.find(texture_id);
	if (it != textures_.end()) {

). So the implementation on Linux lacks this synchronization on textures management.

We add the synchronization by declaring a GMutex, which is a mutex provided by GTK, in this PR. It can make g_hash_table thread-safe in different threads(ui, raster, etc).

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[robert-ancell]

Change looks good, thanks! Just some details.

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Changes reported for pull request #40478 at sha d27e24e

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Changes reported for pull request #40478 at sha 665e35a

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Golden file changes are available for triage from new commit, Click here to view.

Changes reported for pull request #40478 at sha 2f14fa7

[robert-ancell]

Changes look great now, thanks!