Adding sRGB colors to ImGui style.

[RisingLiberty]

This is a pull request to enable SRGB colors within ImGui for all 3 styles (dark, light, classic)
All linear colors have been converted to sRGB formats. The reason sRGB colors are hardcoded instead of calculated at runtime from the linear colors is for performance reasons. This brings in extra maintance as when the default style colors are changed, both the linear and sRGB values need updating.

This PR got created to resolve the issue raised in #578

To enable the sRGB colors, simply add ImGuiConfigFlags_IsSRGB to the config flags and update the style

ImGuiIO& io = ImGui::GetIO();
io.ConfigFlags |= ImGuiConfigFlags_IsSRGB;
ImGui::StyleColorsDark();

Normal dark theme

sRGB dark theme before change

sRGB dark theme after change

[RisingLiberty]

This can probably be merged with #2943

[adv-sw]

this doesn't degamma color wheels, only theme colors so is an incomplete solution.

here's a generic, dynamic fix to the issue (dx12 only - other backends can be upgraded in a similar fashion)

#7904