cube: Fix new validation error

As of Vulkan-ValidationLayers commit 2872f4a, the layers now check that
image layouts specified in descriptors match the current image layout
state.  LAYOUT_GENERAL is not a wildcard for this matching.

Change the image layouts for the combined image sampler descriptors
from LAYOUT_GENERAL to LAYOUT_SHADER_READ_ONLY_OPTIMAL.
diff --git a/cube/cube.cpp b/cube/cube.cpp
index 65cce40..6ff1190 100644
--- a/cube/cube.cpp
+++ b/cube/cube.cpp
@@ -1825,7 +1825,7 @@
     for (uint32_t i = 0; i < texture_count; i++) {
         tex_descs[i].setSampler(textures[i].sampler);
         tex_descs[i].setImageView(textures[i].view);
-        tex_descs[i].setImageLayout(vk::ImageLayout::eGeneral);
+        tex_descs[i].setImageLayout(vk::ImageLayout::eShaderReadOnlyOptimal);
     }
 
     vk::WriteDescriptorSet writes[2];