Vk: interestingly enough, DescriptorSetLayout can be empty.

I wanted to add asserts for this but seems like it's allowed. Adding
tests for that instead, then.

src/Magnum/Vk/DescriptorSetLayoutCreateInfo.h

@@ -134,8 +134,8 @@ class MAGNUM_VK_EXPORT DescriptorSetLayoutBinding {
          *      binding in a shader
          * @param descriptorType    Descriptor type
          * @param descriptorCount   Number of descriptors contained in the
-         *      binding. Has to be at least @cpp 1 @ce. If the shader binding
-         *      is not an array, use @cpp 1 @ce.
+         *      binding. If the shader binding is not an array, use @cpp 1 @ce,
+         *      zero is allowed as well.
          * @param stages            Shader stages that access the binding.
          *      Use @cpp ~Vk::ShaderStages{} @ce to specify that all stages
          *      may access the binding.
@@ -163,7 +163,7 @@ class MAGNUM_VK_EXPORT DescriptorSetLayoutBinding {
          * @param descriptorType    Descriptor type. Should be either
          *      @ref DescriptorType::Sampler or
          *      @ref DescriptorType::CombinedImageSampler.
-         * @param immutableSamplers Immutable samplers
+         * @param immutableSamplers Immutable samplers. Allowed to be empty.
          * @param stages            Shader stages that access the binding.
          *      Use @cpp ~Vk::ShaderStages{} @ce to specify that all stages
          *      may access the binding.
@@ -283,8 +283,8 @@ class MAGNUM_VK_EXPORT DescriptorSetLayoutCreateInfo {
 
         /**
          * @brief Constructor
-         * @param bindings      Descriptor set layout bindings. At least one
-         *      binding has to be present.
+         * @param bindings      Descriptor set layout bindings. Allowed to be
+         *      empty.
          * @param flags         Descriptor set layout creation flags
          *
          * The following @type_vk{DescriptorSetLayoutCreateInfo} fields are

src/Magnum/Vk/Test/DescriptorSetLayoutVkTest.cpp

@@ -34,6 +34,8 @@ struct DescriptorSetLayoutVkTest: VulkanTester {
     explicit DescriptorSetLayoutVkTest();
 
     void construct();
+    void constructEmpty();
+    void constructEmptyBinding();
     void constructMove();
 
     void wrap();
@@ -41,6 +43,8 @@ struct DescriptorSetLayoutVkTest: VulkanTester {
 
 DescriptorSetLayoutVkTest::DescriptorSetLayoutVkTest() {
     addTests({&DescriptorSetLayoutVkTest::construct,
+              &DescriptorSetLayoutVkTest::constructEmpty,
+              &DescriptorSetLayoutVkTest::constructEmptyBinding,
               &DescriptorSetLayoutVkTest::constructMove,
 
               &DescriptorSetLayoutVkTest::wrap});
@@ -59,6 +63,32 @@ void DescriptorSetLayoutVkTest::construct() {
     CORRADE_VERIFY(true);
 }
 
+void DescriptorSetLayoutVkTest::constructEmpty() {
+    {
+        /* Although rather weird, the spec allows this */
+        DescriptorSetLayout layout{device(), DescriptorSetLayoutCreateInfo{}};
+        CORRADE_VERIFY(layout.handle());
+        CORRADE_COMPARE(layout.handleFlags(), HandleFlag::DestroyOnDestruction);
+    }
+
+    /* Shouldn't crash or anything */
+    CORRADE_VERIFY(true);
+}
+
+void DescriptorSetLayoutVkTest::constructEmptyBinding() {
+    {
+        /* Also weird, but the spec *also* allows this */
+        DescriptorSetLayout layout{device(), DescriptorSetLayoutCreateInfo{
+            {{15, DescriptorType::UniformBuffer, 0}}
+        }};
+        CORRADE_VERIFY(layout.handle());
+        CORRADE_COMPARE(layout.handleFlags(), HandleFlag::DestroyOnDestruction);
+    }
+
+    /* Shouldn't crash or anything */
+    CORRADE_VERIFY(true);
+}
+
 void DescriptorSetLayoutVkTest::constructMove() {
     DescriptorSetLayout a{device(), DescriptorSetLayoutCreateInfo{
         {{15, DescriptorType::UniformBuffer}}