Enable descriptor_count() setter on ash::vk::WriteDescriptorSet

[MarijnS95]

Fixes #806

This parameter is not only used for the length of pImageInfo, pBufferInfo or pTexelBufferView, but also matching the value of dataSize when VkWriteDescriptorSetInlineUniformBlock is appended in pNext, or the value of accelerationStructureCount when VkWriteDescriptorSetAccelerationStructureKHR is in pNext. Having the count setter directly avaialble makes builder code more natural, instead of having to use a mut variable and manually assign .descriptor_count = xx.len(); afterwards.

https://registry.khronos.org/vulkan/specs/1.3-extensions/man/html/VkWriteDescriptorSet.html

[Ralith]

There's always FRU syntax, but yeah.

[MarijnS95]

Sure, but it feels as hidden away as the mut variant, while in this case there are multiple obvious uses where this member needs to be used in isolation.

Combined FRU syntax isn't too nice to read in my opinion:

let write_descriptor_set = vk::WriteDescriptorSet {
    descriptor_count: as.acceleration_structure_count,
    ..Default::default()
}
.dst_set(...)
.dst_binding(0)
.descriptor_type(vk::DescriptorType::ACCELERATION_STRUCTURE_KHR)
.push_next(&mut as);

Unless one moves all member assignments (PODs) into FRU, except .push_next() which requires casting and should update the lifetime.