[wpigui] Refactor texture handling

The platform-specific code now only has create, update, and delete texture.
Image reading functions have been moved to common code.

Also add pixel data functions and image data functions in addition to image
file loading.

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

@@ -277,7 +277,10 @@ bool HALSimGui::Initialize() {
   return true;
 }
 
-void HALSimGui::Main(void*) { gui::Main(); }
+void HALSimGui::Main(void*) {
+  gui::Main();
+  gui::DestroyContext();
+}
 
 void HALSimGui::Exit(void*) { gui::Exit(); }