vkformat: fix features fetch and guard code better
vkformat: use VK_KHR_get_physical_device_properties2 to guard and VK_KHR_format_feature_flags2 rather than a meaningless version handling.
Also the patch brings back the usage of features2 for video extensions, use KHR version to support the guarded symbols.
Feature flags2 are fetch through VkFormatProperties3KHR and needs more guards checking. Moved out all the feature flags fetching to another function for clarity.