[OpenGL] Explicitly set ImGui's context and treat global context as foreign

Apps might be using Dear ImGui so keep our stuff separate from theirs.

Issue #107

src/gl/imgui_impl_opengl3.cpp

@@ -77,6 +77,8 @@
 void* get_glx_proc_address(const char* name);
 void (*glClipControl)(int origin, int depth);
 
+namespace MangoHud {
+
 // Desktop GL 3.2+ has glDrawElementsBaseVertex() which GL ES and WebGL don't have.
 #if defined(IMGUI_IMPL_OPENGL_ES2) || defined(IMGUI_IMPL_OPENGL_ES3) || !defined(GL_VERSION_3_2)
 #define IMGUI_IMPL_OPENGL_MAY_HAVE_VTX_OFFSET   0
@@ -625,3 +627,4 @@ void    ImGui_ImplOpenGL3_DestroyDeviceObjects()
 
     ImGui_ImplOpenGL3_DestroyFontsTexture();
 }
+}

src/gl/imgui_impl_opengl3.h

@@ -23,6 +23,8 @@
 
 #pragma once
 
+namespace MangoHud {
+
 // Backend API
 IMGUI_IMPL_API bool     ImGui_ImplOpenGL3_Init(const char* glsl_version = nullptr);
 IMGUI_IMPL_API void     ImGui_ImplOpenGL3_Shutdown();
@@ -34,3 +36,5 @@ IMGUI_IMPL_API bool     ImGui_ImplOpenGL3_CreateFontsTexture();
 IMGUI_IMPL_API void     ImGui_ImplOpenGL3_DestroyFontsTexture();
 IMGUI_IMPL_API bool     ImGui_ImplOpenGL3_CreateDeviceObjects();
 IMGUI_IMPL_API void     ImGui_ImplOpenGL3_DestroyDeviceObjects();
+
+}

src/gl/inject.cpp

@@ -24,6 +24,8 @@
 #include <chrono>
 #include <iomanip>
 
+using namespace MangoHud;
+
 #define EXPORT_C_(type) extern "C" __attribute__((__visibility__("default"))) type
 
 EXPORT_C_(void *) glXGetProcAddress(const unsigned char* procName);
@@ -153,6 +155,9 @@ void imgui_create(void *ctx)
         state.font1 = io.Fonts->AddFontFromMemoryCompressedBase85TTF(ttf_compressed_base85, font_size * 0.55, &font_cfg, glyph_ranges);
     }
     sw_stats.font1 = state.font1;
+
+    // Reset global context to null, might clash with apps that use Dear ImGui
+    ImGui::SetCurrentContext(nullptr);
 }
 
 void imgui_shutdown()
@@ -162,6 +167,7 @@ void imgui_shutdown()
 #endif
 
     if (state.imgui_ctx) {
+        ImGui::SetCurrentContext(state.imgui_ctx);
         ImGui_ImplOpenGL3_Shutdown();
         ImGui::DestroyContext(state.imgui_ctx);
         state.imgui_ctx = nullptr;
@@ -183,9 +189,12 @@ void imgui_set_context(void *ctx)
 
 void imgui_render()
 {
-    if (!ImGui::GetCurrentContext())
+    if (!state.imgui_ctx)
         return;
 
+    ImGuiContext *saved_ctx = ImGui::GetCurrentContext();
+    ImGui::SetCurrentContext(state.imgui_ctx);
+
     // check which one is affected by window resize and use that
     GLVec vp; glGetIntegerv (GL_VIEWPORT, vp.v);
     GLVec sb; glGetIntegerv (GL_SCISSOR_BOX, sb.v);
@@ -224,6 +233,7 @@ void imgui_render()
 
     ImGui::Render();
     ImGui_ImplOpenGL3_RenderDrawData(ImGui::GetDrawData());
+    ImGui::SetCurrentContext(saved_ctx);
 }
 
 void* get_proc_address(const char* name) {