ImGuiHelper can construct with a context (#3753)

* ImGuiHelper supports client-provided context

this lets impress load >1 embedded  font into the font table

* ImGuiHelper supports client-provided context

this lets impress load >1 embedded  font into the font table

Co-authored-by: Philip Rideout <philiprideout@gmail.com>

libs/filagui/include/filagui/ImGuiHelper.h

@@ -47,7 +47,8 @@ public:
     using Callback = std::function<void(filament::Engine*, filament::View*)>;
 
     // The constructor creates its own Scene and places it in the given View.
-    ImGuiHelper(filament::Engine* engine, filament::View* view, const utils::Path& fontPath);
+    ImGuiHelper(filament::Engine* engine, filament::View* view, const utils::Path& fontPath,
+            ImGuiContext* imGuiContext = nullptr);
     ~ImGuiHelper();
 
     // Informs ImGui of the current display size, as well as a scaling factor when scissoring.

libs/filagui/src/ImGuiHelper.cpp

@@ -45,9 +45,10 @@ namespace filagui {
 
 #include "generated/resources/filagui_resources.h"
 
-ImGuiHelper::ImGuiHelper(Engine* engine, filament::View* view, const Path& fontPath) :
-        mEngine(engine), mView(view), mScene(engine->createScene()),
-        mImGuiContext(ImGui::CreateContext()) {
+ImGuiHelper::ImGuiHelper(Engine* engine, filament::View* view, const Path& fontPath, 
+        ImGuiContext *imGuiContext) 
+        : mEngine(engine), mView(view), mScene(engine->createScene()),
+        mImGuiContext(imGuiContext ? imGuiContext : ImGui::CreateContext()) {
     ImGuiIO& io = ImGui::GetIO();
 
     // Create a simple alpha-blended 2D blitting material.
@@ -63,7 +64,7 @@ ImGuiHelper::ImGuiHelper(Engine* engine, filament::View* view, const Path& fontP
     createAtlasTexture(engine);
 
     // For proggy, switch to NEAREST for pixel-perfect text.
-    if (fontPath.isEmpty()) {
+    if (fontPath.isEmpty() && !imGuiContext) {
         TextureSampler sampler(MinFilter::NEAREST, MagFilter::NEAREST);
         mMaterial->setDefaultParameter("albedo", mTexture, sampler);
     }