Allow user to override ImTextureId. (#1641)

imgui.h

@@ -74,11 +74,14 @@ struct ImGuiListClipper;            // Helper to manually clip large list of ite
 struct ImGuiPayload;                // User data payload for drag and drop operations
 struct ImGuiContext;                // ImGui context (opaque)
 
+#ifndef ImTextureID
+typedef void* ImTextureID;          // user data to identify a texture (this is whatever to you want it to be! read the FAQ about ImTextureID in imgui.cpp)
+#endif
+
 // Typedefs and Enumerations (declared as int for compatibility and to not pollute the top of this file)
 typedef unsigned int ImU32;         // 32-bit unsigned integer (typically used to store packed colors)
 typedef unsigned int ImGuiID;       // unique ID used by widgets (typically hashed from a stack of string)
 typedef unsigned short ImWchar;     // character for keyboard input/display
-typedef void* ImTextureID;          // user data to identify a texture (this is whatever to you want it to be! read the FAQ about ImTextureID in imgui.cpp)
 typedef int ImGuiCol;               // enum: a color identifier for styling     // enum ImGuiCol_
 typedef int ImGuiCond;              // enum: a condition for Set*()             // enum ImGuiCond_
 typedef int ImGuiKey;               // enum: a key identifier (ImGui-side enum) // enum ImGuiKey_