Added own hacky mangohud ebuilds

I cloned the upstream repo myself to pull the meson subprojects, and
tarballed them. They are then extracted in src_prepare() to the right
right directory to build mangohud successfully.

This was to fix new vulkan headers breaking mangohud, so the subprojects
are used instead.

games-util/mangohud/files/mangohud-0.6.6-meson-fix-imgui-dep.patch

@@ -0,0 +1,14 @@
+# We provide media-libs/imgui in guru
+
+--- a/meson.build
++++ b/meson.build
+@@ -251,8 +251,7 @@ elif sizeof_ptr == 4
+   pre_args += '-DMANGOHUD_ARCH="32bit"'
+ endif
+
+-dearimgui_sp = subproject('imgui', default_options: imgui_options)
+-dearimgui_dep = dearimgui_sp.get_variable('imgui_dep')
++dearimgui_dep = dependency('imgui')
+
+ spdlog_dep = cpp.find_library('spdlog', required: get_option('use_system_spdlog'))
+ if not spdlog_dep.found()