[simgui] Add GUI context getter hooks (#8127)

This enables GUI libraries to be linked statically with shared context.

simulation/halsim_gui/src/main/native/cpp/main.cpp

@@ -65,6 +65,12 @@ __declspec(dllexport)
       reinterpret_cast<void*>((AddGuiEarlyExecuteFn)&AddGuiEarlyExecute));
   HAL_RegisterExtension(HALSIMGUI_EXT_GUIEXIT,
                         reinterpret_cast<void*>((GuiExitFn)&GuiExit));
+  HAL_RegisterExtension(
+      HALSIMGUI_EXT_GETGUICONTEXT,
+      reinterpret_cast<void*>((GetGuiContextFn)&gui::GetCurrentContext));
+  HAL_RegisterExtension(
+      HALSIMGUI_EXT_GETGLASSCONTEXT,
+      reinterpret_cast<void*>((GetGlassContextFn)&glass::GetCurrentContext));
 
   HALSimGui::GlobalInit();
   DriverStationGui::GlobalInit();
@@ -158,6 +164,9 @@ __declspec(dllexport)
                        ImGuiConfigFlags_DockingEnable)) {
     return 0;
   }
+  HAL_RegisterExtension(
+      HALSIMGUI_EXT_GETIMGUICONTEXT,
+      reinterpret_cast<void*>((GetImguiContextFn)&ImGui::GetCurrentContext));
   HAL_RegisterExtensionListener(
       nullptr, [](void*, const char* name, void* data) {
         if (std::string_view{name} == "ds_socket") {

simulation/halsim_gui/src/main/native/include/HALSimGuiExt.h

@@ -6,6 +6,16 @@
 
 #include <functional>
 
+struct ImGuiContext;
+
+namespace glass {
+class Context;
+}  // namespace glass
+
+namespace wpi::gui {
+struct Context;
+}  // namespace wpi::gui
+
 namespace halsimgui {
 
 // These functions can be used to hook into the GUI, and can be accessed
@@ -23,4 +33,13 @@ using AddGuiEarlyExecuteFn = void (*)(std::function<void()> execute);
 #define HALSIMGUI_EXT_GUIEXIT "halsimgui::GuiExit"
 using GuiExitFn = void (*)();
 
+#define HALSIMGUI_EXT_GETIMGUICONTEXT "halsimgui::GetImguiContext"
+using GetImguiContextFn = ImGuiContext* (*)();
+
+#define HALSIMGUI_EXT_GETGUICONTEXT "halsimgui::GetGuiContext"
+using GetGuiContextFn = wpi::gui::Context* (*)();
+
+#define HALSIMGUI_EXT_GETGLASSCONTEXT "halsimgui::GetGlassContext"
+using GetGlassContextFn = glass::Context* (*)();
+
 }  // namespace halsimgui