Textures: extract ImTextureDataQueueUpload() out of ImFontAtlasTextureBlockQueueUpload() (#8465)

Not atlas specific, in support of texture system.

imgui.cpp

@@ -9013,7 +9013,7 @@ ImFont* ImGui::GetDefaultFont()
     return g.IO.FontDefault ? g.IO.FontDefault : atlas->Fonts[0];
 }
 
-// EXPERIMENTAL: DO NOT USE YET.
+// EXPERIMENTAL. Use ImTextureDataQueueUpload() to queue updates.
 void ImGui::RegisterUserTexture(ImTextureData* tex)
 {
     ImGuiContext& g = *GImGui;