Support Descriptor Indexing in Vulkan

[Erfan-Ahmadi]

I believe It's the case where descriptorIndexing device extension is enabled and also physDevDescriptorIndexingFeatures.descriptorBindingVariableDescriptorCount is equal to VK_TRUE
meaning you'd enable variable sized descriptor arrays for shaders. (must be the last binding in the set)

So you don't know your the length of your variable-sized descriptor up to the point you actually want to allocate it's descriptorSet from the descriptorPool. This is what this struct is for; and It's basically asking for the "Count" of the "VariableCount" descriptors.

descriptorBindingVariableDescriptorCount indicates whether the implementation supports descriptor sets with a variable-sized last binding. If this feature is not enabled, VK_DESCRIPTOR_BINDING_VARIABLE_DESCRIPTOR_COUNT_BIT must not be used.

In order to expose thisIGPUDescriptorSetLayout::SBinding should have an enum (a flag) related to this Vulkan Enum:

typedef enum VkDescriptorBindingFlagBits {
    VK_DESCRIPTOR_BINDING_UPDATE_AFTER_BIND_BIT = 0x00000001,
    VK_DESCRIPTOR_BINDING_UPDATE_UNUSED_WHILE_PENDING_BIT = 0x00000002,
    VK_DESCRIPTOR_BINDING_PARTIALLY_BOUND_BIT = 0x00000004,
    VK_DESCRIPTOR_BINDING_VARIABLE_DESCRIPTOR_COUNT_BIT = 0x00000008,
.
.
.
} VkDescriptorBindingFlagBits;

In order for it to work you should also give the binding the correct flag when creating the DescriptorSetLayout. VkDescriptorSetLayoutBindingFlagsCreateInfoEXT in the pNext chain of VkDescriptorSetLayoutCreateInfo

Also we can do some validation ourselves here: for example check if the binding with the flagVARIABLE_DESCRIPTOR_COUNT_BIT is the last binding number in the set.

as mentioned here in the commetns https://github.com/Devsh-Graphics-Programming/Nabla/blob/6634885ebe66f4e9ba1657631f0902e0773ff7ba/src/nbl/video/CVKLogicalDevice.h#L1003

Originally posted by @Erfan-Ahmadi in https://github.com/Devsh-Graphics-Programming/Nabla/pull/160#discussion_r690916570

[devshgraphicsprogramming]

if the variable count binding needs to be the last one, no point having that flag on the binding, better have it on the entire set :)