vulkan: Add vkCmdPushConstants entry point

This patch fixes LunarXchange issue #84.
Also adds corresponding stub entry point to the Intel
sample driver.
diff --git a/vulkan.py b/vulkan.py
index da23d5d..71cbe84 100755
--- a/vulkan.py
+++ b/vulkan.py
@@ -1013,6 +1013,14 @@
             [Param("VkCmdBuffer", "cmdBuffer"),
              Param("VkRenderPassContents", "contents")]),
 
+        Proto("void", "CmdPushConstants",
+            [Param("VkCmdBuffer", "cmdBuffer"),
+             Param("VkPipelineLayout", "layout"),
+             Param("VkShaderStageFlags", "stageFlags"),
+             Param("uint32_t", "start"),
+             Param("uint32_t", "length"),
+             Param("const void*", "values")]),
+
         Proto("void", "CmdEndRenderPass",
             [Param("VkCmdBuffer", "cmdBuffer")]),