vsgImGui refactor adds ImGui texture support, MSAA and update to ImGui v189.4

[vsg-dev]

I have just merge a refactor of vsgImGui with master that improves it's capabilities and flexibility, the head line changes are:

• Refactor of vsg::RenderImGui to become a vsg::Group so that ImGui elements you add can be done by subclassing from vsg::Command and overriding the compile() and record() methods as required. An adapter class wraps the legacy std::function<bool()> to keep existing applications compiling.
• Introduction of a vsgImGui::Texture class that provides the required DescriptorSet/DescrptorImage wrapper around vsg::Data image data in a form that makes it convenient to use with ImGui::Image(...) as well as all the back-end required to require to get the data to the GPU and encapsulated int the required vlkImage/vkImageView/vkDescriptorImage/DescriptorSet.
• Added support for passing on the RenderPass samples value to ImGui to ensure that applications that use MSAA work automatically with ImGui rendering widgets.
• Updated the ImGui version to the latest v189.4 release
• Updated the ImPlot version to master to ensure it builds with the latest ImGui changes.
• Updated the vsgImGui version from 0.0.0 to 0.1.0 to signify the change in API and capabilities.

The changes made to vsgImgui are: vsg-dev/vsgImGui#47

To test out the new capabilities and to illustrate how to use the new vsg::Command subclass approach the vsgimgui_example has been updated to include:

• --sample <num_samples> command line support to enable MSAA
• Use of vsgImGui::Texture with ImGui::Image to render the VSG logo in full and cropped form, as well as a button.
• Use of subclass from vsg::Command with an override of the Command::compile(Context&) method require to compile the texture prior to use, and override of Command::record(RecordTraversal&) const to do the ImGui calls. This replaces the previous std::function<bool()> usage that is deprecated but it still supported by vsgImGui.

The changes to vsgExaples are: vsg-dev/vsgExamples#218

When you run:

vsgimgui_example models/openstreetmap.vsgt

You should now see:

Thanks to @timoore for providing the initial ImGui texturing support, this spurred the refactor of vsgImGui to make it more flexible.

I'm now feel that vsgImGui is getting close to a functional enough state to tag it's 1.0.0. I'm not yet happy with the large pre-allocation of descriptors/set pool sizes just "in case" they might be needed so that's something I want to revisit.

The vsgImGui::Texture example in vsgimgui_example is geared around a static vsg::Data, but I think it should also work for dynamic vsg::Data, and should be adaptable to use a user defined vsgImGui::Texture::descriptorSet which I'm hoping should make rendering texture possible - i.e. you render a 3d scene to image that you then define a descriptorSet for so it can be used on the ImGui side. I think these two usage cases would make great dedicated examples to sit alongside the present vsgimgui_example, so if members want a challenge for themselves please jump in :-)