ImFontAtlas: move implicit AddFontDefault call to Build() function + remove unnecessary asserts in backend.

backends/imgui_impl_android.cpp

@@ -157,7 +157,6 @@ void ImGui_ImplAndroid_Shutdown()
 void ImGui_ImplAndroid_NewFrame()
 {
     ImGuiIO& io = ImGui::GetIO();
-    IM_ASSERT(io.Fonts->IsBuilt() && "Font atlas not built! It is generally built by the renderer backend. Missing call to renderer _NewFrame() function? e.g. ImGui_ImplOpenGL3_NewFrame().");
 
     // Process queued key events
     // FIXME: This is a workaround for multiple key event actions occurring at once (see above) and can be removed once we use upcoming input queue.

backends/imgui_impl_glfw.cpp

@@ -354,7 +354,6 @@ static void ImGui_ImplGlfw_UpdateGamepads()
 void ImGui_ImplGlfw_NewFrame()
 {
     ImGuiIO& io = ImGui::GetIO();
-    IM_ASSERT(io.Fonts->IsBuilt() && "Font atlas not built! It is generally built by the renderer backend. Missing call to renderer _NewFrame() function? e.g. ImGui_ImplOpenGL3_NewFrame().");
 
     // Setup display size (every frame to accommodate for window resizing)
     int w, h;

backends/imgui_impl_sdl.cpp

@@ -351,7 +351,6 @@ static void ImGui_ImplSDL2_UpdateGamepads()
 void ImGui_ImplSDL2_NewFrame(SDL_Window* window)
 {
     ImGuiIO& io = ImGui::GetIO();
-    IM_ASSERT(io.Fonts->IsBuilt() && "Font atlas not built! It is generally built by the renderer backend. Missing call to renderer _NewFrame() function? e.g. ImGui_ImplOpenGL3_NewFrame().");
 
     // Setup display size (every frame to accommodate for window resizing)
     int w, h;

backends/imgui_impl_win32.cpp

@@ -267,7 +267,6 @@ static void ImGui_ImplWin32_UpdateGamepads()
 void    ImGui_ImplWin32_NewFrame()
 {
     ImGuiIO& io = ImGui::GetIO();
-    IM_ASSERT(io.Fonts->IsBuilt() && "Font atlas not built! It is generally built by the renderer backend. Missing call to renderer _NewFrame() function? e.g. ImGui_ImplOpenGL3_NewFrame().");
 
     // Setup display size (every frame to accommodate for window resizing)
     RECT rect = { 0, 0, 0, 0 };

imgui.cpp

@@ -7173,8 +7173,7 @@ static void ImGui::ErrorCheckNewFrameSanityChecks()
     IM_ASSERT((g.IO.DeltaTime > 0.0f || g.FrameCount == 0)              && "Need a positive DeltaTime!");
     IM_ASSERT((g.FrameCount == 0 || g.FrameCountEnded == g.FrameCount)  && "Forgot to call Render() or EndFrame() at the end of the previous frame?");
     IM_ASSERT(g.IO.DisplaySize.x >= 0.0f && g.IO.DisplaySize.y >= 0.0f  && "Invalid DisplaySize value!");
-    IM_ASSERT(g.IO.Fonts->Fonts.Size > 0                                && "Font Atlas not built. Did you call io.Fonts->GetTexDataAsRGBA32() / GetTexDataAsAlpha8()?");
+    IM_ASSERT(g.IO.Fonts->IsBuilt()                                     && "Font Atlas not built! Make sure you called ImGui_ImplXXXX_NewFrame() function for renderer backend, which should call io.Fonts->GetTexDataAsRGBA32() / GetTexDataAsAlpha8()");
-    IM_ASSERT(g.IO.Fonts->Fonts[0]->IsLoaded()                          && "Font Atlas not built. Did you call io.Fonts->GetTexDataAsRGBA32() / GetTexDataAsAlpha8()?");
     IM_ASSERT(g.Style.CurveTessellationTol > 0.0f                       && "Invalid style setting!");
     IM_ASSERT(g.Style.CircleTessellationMaxError  > 0.0f                && "Invalid style setting!");
     IM_ASSERT(g.Style.Alpha >= 0.0f && g.Style.Alpha <= 1.0f            && "Invalid style setting!"); // Allows us to avoid a few clamps in color computations

imgui_draw.cpp

@@ -2033,11 +2033,7 @@ void    ImFontAtlas::GetTexDataAsAlpha8(unsigned char** out_pixels, int* out_wid
 {
     // Build atlas on demand
     if (TexPixelsAlpha8 == NULL)
-    {
-        if (ConfigData.empty())
-            AddFontDefault();
         Build();
-    }
 
     *out_pixels = TexPixelsAlpha8;
     if (out_width) *out_width = TexWidth;
@@ -2258,6 +2254,10 @@ bool    ImFontAtlas::Build()
 {
     IM_ASSERT(!Locked && "Cannot modify a locked ImFontAtlas between NewFrame() and EndFrame/Render()!");
 
+    // Default font is none are specified
+    if (ConfigData.Size == 0)
+        AddFontDefault();
+
     // Select builder
     // - Note that we do not reassign to atlas->FontBuilderIO, since it is likely to point to static data which
     //   may mess with some hot-reloading schemes. If you need to assign to this (for dynamic selection) AND are