RGBA format and fixed RGB debayer with vertical flip

[jkevin]

A couple of small improvements I made while implementing a DirectShow filter based on this code. The RGB debayer function was extended to RGBA using a template so that two versions of the function are compiled. This was done with the idea that having a static number of channels may help the compiler optimize this function better and also it avoids having to check if the alpha channel should be set for every pixel.