Minor SDK-0.2 fixes for "tri"

Changes to "tri" are similar to the fixes for "cube":

1) WSI function were being called with an uninitialized pPlatformWindow.
Fixed by splitting demo_init_vk into two functions.

2) VkAttachmentViewCreateInfo.format wasn't being set.

3) VkShaderCreateInfo.pName wasn't being set.

4) demo->cmdPool was being leaked.
diff --git a/demos/cube.c b/demos/cube.c
index 3b41804..73562ec 100644
--- a/demos/cube.c
+++ b/demos/cube.c
@@ -1426,6 +1426,7 @@
 
         shaderCreateInfo.flags = 0;
         shaderCreateInfo.module = shaderModule;
+        shaderCreateInfo.pName = "main";
         err = vkCreateShader(demo->device, &shaderCreateInfo, &shader);
     } else {
         // Create fake SPV structure to feed GLSL
@@ -1447,6 +1448,7 @@
 
         shaderCreateInfo.flags = 0;
         shaderCreateInfo.module = shaderModule;
+        shaderCreateInfo.pName = "main";
         err = vkCreateShader(demo->device, &shaderCreateInfo, &shader);
     }
     return shader;