bug 15022: Need a way to specify dummy descriptors for unused bindings (WIP)

Add binding to VkDescriptorSetLayoutBinding.

https://cvs.khronos.org/bugzilla/show_bug.cgi?id=15022
diff --git a/demos/cube.c b/demos/cube.c
index 29c1c8e..dd8cbd5 100644
--- a/demos/cube.c
+++ b/demos/cube.c
@@ -1290,12 +1290,14 @@
 {
     const VkDescriptorSetLayoutBinding layout_bindings[2] = {
         [0] = {
+            .binding = 0,
             .descriptorType = VK_DESCRIPTOR_TYPE_UNIFORM_BUFFER,
             .arraySize = 1,
             .stageFlags = VK_SHADER_STAGE_VERTEX_BIT,
             .pImmutableSamplers = NULL,
         },
         [1] = {
+            .binding = 1,
             .descriptorType = VK_DESCRIPTOR_TYPE_COMBINED_IMAGE_SAMPLER,
             .arraySize = DEMO_TEXTURE_COUNT,
             .stageFlags = VK_SHADER_STAGE_FRAGMENT_BIT,