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

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

wpigui/src/main/native/cpp/wpigui.cpp

@@ -169,6 +169,14 @@ void gui::DestroyContext() {
   gContext = nullptr;
 }
 
+Context* gui::GetCurrentContext() {
+  return gContext;
+}
+
+void gui::SetCurrentContext(Context* context) {
+  gContext = context;
+}
+
 static void UpdateFontScale() {
   // Scale based on OS window content scaling
   float windowScale = 1.0;

wpigui/src/main/native/include/wpigui.h

@@ -14,6 +14,8 @@ extern "C" struct GLFWwindow;
 
 namespace wpi::gui {
 
+struct Context;
+
 /**
  * Creates GUI context.  Must be called prior to calling any other functions.
  */
@@ -24,6 +26,20 @@ void CreateContext();
  */
 void DestroyContext();
 
+/**
+ * Gets GUI context.
+ *
+ * @return context
+ */
+Context* GetCurrentContext();
+
+/**
+ * Sets GUI context.
+ *
+ * @param context context
+ */
+void SetCurrentContext(Context* context);
+
 /**
  * Initializes the GUI.
  *