…ction in the fragment shader

API changes:
- 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(...) before rendering)
Concerning color correction:
- The default behavior of imgui_impl_vulkan does not change (no color correction is applied).
- A color correction method is decided at pipeline compile time (through a vulkan specialization constant)
- Color correction parameters can either be set at pipeline compile time (static mode), or through push_constant (dynamic mode, default)
- A gamma correction mode is implemented (and an extra alpha gamma correction).
- New color modes can easily be added.

# Conflicts:
#	backends/imgui_impl_vulkan.cpp

… not working yet!)