Handle gcc 7 warnings

Handle gcc 7 warnings

- "-Wimplicit-fallthrough" warns on case conditions that
  fallthrough. GCC does have a way to accept conditions that have a
  comment about falling through inside the cases, but the codebase has
  other cases where a single comment explains the fallthrough for the
  whole switch. This warning is being ignored. It was not possible to
  use "-Wno-error=WARNING" because older GCCs fail -- but they do
  accept "-Wno-WARNING" where WARNING is unknown (since at least gcc
  4.6.4).

- "-Wint-in-bool-context" warns (among other things) about result of
  multiplication being used in boolean context. Code was changed to
  use de::max() instead.

- "-Wmaybe-uninitialized" warned about some attributes of a struct not
  being initialized depending on the case. Code was changed to
  initialize the struct in question.

Components: Framework, Vulkan

Change-Id: Iabb26f01e047353c6e2a704ab27b673e85b185be