Overlay: Changes as per PR-comments

* Make cpp-local functions static so they are local file/translation unit
scope.
* Remove unused variables uiAudioCount and bVideoHooked
* Comment extern declarations, where there definitions are located.
* Improve constant naming JUMPOP_OFFSET => JMP_OP_SIZE.
* Remove extern declaration from method definitions.
* Adjust type declaration formatting to typical formatting.
* Use voidFunc typedef to pass around function address pointers.
** C++-style casts instead of C-style raw casts.
* Other minor formatting (/whitespace) adjustments
* Fix log text.
* Improve log text.
* Consistent, improved varnaming in d3d9.cpp on address calculation;
naming fn and base.