Fix scene cache output determinism

Two of our structures had uninitialized padding space; as a result, some
of the data that we were writing to scene cache would have uninitialized
bytes which meant the cache was not reproducible run to run.

The padding amount would be conditional on the contents of the rest of
the structure, so this is not ideal for maintenance but for now this
fixes the problem and makes it easier to debug.

Author: zeux

src/scene.h

@@ -32,6 +32,8 @@ struct alignas(16) Material
 	vec4 diffuseFactor;
 	vec4 specularFactor;
 	vec3 emissiveFactor;
+
+	uint32_t padding; // TODO: this is fragile
 };
 
 struct alignas(16) MeshDraw
@@ -73,6 +75,8 @@ struct alignas(16) Mesh
 
 	uint32_t lodCount;
 	MeshLod lods[8];
+
+	uint32_t padding; // TODO: this is fragile
 };
 
 struct Geometry