Vulkan: Re-Create main window pipeline

[SuperRonan]

• Added ImGui_ImplVulkan_ReCreateMainPipeline(...) to explicitly re-create the main window pipeline (when some of its properties are changed).
• ImGui_ImplVulkan_ReCreateMainPipeline(...) does not implicitly use ImGui_ImplVulkan_InitInfo::PipelineRenderingCreateInfo, but a function parameter.
• The main window pipeline is created only if possible during ImGui_ImplVulkan_Init(...) (if a render pass or rendering info are given), else it should be created with ImGui_ImplVulkan_ReCreateMainPipeline(...) R.
• The rendering does seem to work properly only UNORM render target (with other formats, the GUI is missing a color correction). This will be fixed in a future PR.

[SuperRonan]

I will also write an extension of these changes for the docking branch.

[SuperRonan]

I also added a optional color correction to the imgui fragment shader. It should not alter the default behavior of imgui_impl_vulkan.

[ocornut]

?

[SuperRonan]

I will re-open it soon (this evening or tomorrow) with more changes (and a docking version), I did not want to burden you a merge of an outdated version.

[ocornut]

You can push or force push to a same PR to update it!

But FYI doing multiple unrelated things in a same PR is the best way to get it to take years to get looked at.

Providing clear rational / proof / exemples for everything would be helpful too. Vulkan related changes are notoriously slow because people seem often happy to not explore all angles. (But an unmerged PR is often better than no PR, as the visibility helps people align toward a solution)

Thanks!