Move extension support check from device creation to vulkan_sample::prepare

[asuessenbach]

Currently, device extension support is checked right before the device creation in Device<bindingType>::init().
Feature support is checked in VulkanSample<bindingType>::prepare().

The VulkanSample<bindingType>::prepare() (correctly) initiates the device creation after the feature support check.. But now, the feature support check is done before the extension support check.
But as the extension support check is the coarser one, it should be done before the feature support check.

That is, the feature support check should be move from device creation to vulkan_sample::prepare, to before the feature support check there.