Clean up depth format selection code

All samples that do depth buffering have code like this:

```
void createDepthResources()
{
vk::Format depthFormat = findDepthFormat();

createImage(swapChainExtent.width, swapChainExtent.height, depthFormat, vk::ImageTiling::eOptimal, vk::ImageUsageFlagBits::eDepthStencilAttachment, vk::MemoryPropertyFlagBits::eDeviceLocal, depthImage, depthImageMemory);
depthImageView = createImageView(depthImage, depthFormat, vk::ImageAspectFlagBits::eDepth);
}

vk::Format findSupportedFormat(const std::vector &candidates, vk::ImageTiling tiling, vk::FormatFeatureFlags features) const
{
for (const auto format : candidates)
{
vk::FormatProperties props = physicalDevice.getFormatProperties(format);

if (tiling == vk::ImageTiling::eLinear && (props.linearTilingFeatures & features) == features)
{
return format;
}
if (tiling == vk::ImageTiling::eOptimal && (props.optimalTilingFeatures & features) == features)
{
return format;
}
}

throw std::runtime_error("failed to find supported format!");
}

[[nodiscard]] vk::Format findDepthFormat() const
{
return findSupportedFormat(
{vk::Format::eD32Sfloat, vk::Format::eD32SfloatS8Uint, vk::Format::eD24UnormS8Uint},
vk::ImageTiling::eOptimal,
vk::FormatFeatureFlagBits::eDepthStencilAttachment);
}

....
// Somewhere in a function A
vk::Format depthFormat = findDepthFormat();
....
// Somewhere in a function B
vk::Format depthFormat = findDepthFormat();
```

* `findDepthFormat` is often called multiple times of just querying once and then storing it.
* `findSupportedFormat` is only ever used in `findDepthFormat`
* `findDepthFormat` is declared after the calling function(s) `createDepthResources`

There is potential here to simplify code and make it easier to follow.