Create ColorPicker shaders statically

[KoBeWi]

As discovered by @pycbouh, the recent inspector slowdows were introduced by #46340. Turns out that each ColorPicker was compiling shaders when it was created, which was terribly slow.

This PR moves the shader compilation to a static method, so it's done only once.

[YuriSizov]

Definitely alleviates some of the slowdown. There are likely some other causes unrelated to #46340 too, but this is good.

[KoBeWi]

Any ideas why the checks failed and how to fix this? Seems like tests have problems with instancing the shader.

[groud]

It seems like there is a segfault. I would try running the unit tests locally and see what causes the bug.

 ERROR: Invalid locale 'C'.
   at: get_language_code (core/string/translation.cpp:957)
ERROR: Unsupported locale 'C', falling back to 'en'.
   at: set_locale (core/string/translation.cpp:980)
scene/resources/shader.cpp:155:59: runtime error: member access within null pointer of type 'struct RenderingServer'
handle_crash: Program crashed with signal 11
Dumping the backtrace. Please include this when reporting the bug on https://github.com/godotengine/godot/issues
[1] ./bin/godot.linuxbsd.tools.64s() [0x1fc8878] (/home/runner/work/godot/godot/platform/linuxbsd/crash_handler_linuxbsd.cpp:54)
[2] /lib/x86_64-linux-gnu/libc.so.6(+0x46210) [0x7f1529b57210] (??:0)
[3] Shader::Shader() (/home/runner/work/godot/godot/scene/resources/shader.cpp:155 (discriminator 1))
[4] Ref<Shader>::instance() (/home/runner/work/godot/godot/./core/object/reference.h:227)
[5] ColorPicker::initialize_shaders() (/home/runner/work/godot/godot/scene/gui/color_picker.cpp:92)
[6] register_scene_types() (/home/runner/work/godot/godot/scene/register_scene_types.cpp:966)
[7] Main::test_setup() (/home/runner/work/godot/godot/main/main.cpp:422)
[8] Main::test_entrypoint(int, char**, bool&) (/home/runner/work/godot/godot/main/main.cpp:486)
[9] ./bin/godot.linuxbsd.tools.64s(main+0x144) [0x1fc5efa] (/home/runner/work/godot/godot/platform/linuxbsd/godot_linuxbsd.cpp:45)
[10] /lib/x86_64-linux-gnu/libc.so.6(__libc_start_main+0xf3) [0x7f1529b380b3] (??:0)
[11] ./bin/godot.linuxbsd.tools.64s(_start+0x2e) [0x1fc5cfe] (??:?)
-- END OF BACKTRACE --

[KoBeWi]

Well I tried and I have no idea. It's like Ref is not initialized, but I tried creating a local variable and it doesn't work either. memnew also doesn't help.

[akien-mga]

The crash happens in the test codepath (Main::test_entrypoint which does not initialize the RenderingServer). Apparently this addition of static Shaders in scene is a first and adds a dependency for the RenderingServer to be fully initialized before nodes, which might be problematic.

[KoBeWi]

Ok, fixed. I also refactored it a bit, because I noticed that other classes create some shaders too.

[akien-mga]

Thanks!