fix ofShader include paths

before this fix, shader #pragma include paths were always relative to current OF_DATA_PATH.

This was not in line with how the c preprocessor resolved relative paths, and it also limited portability of shader modules.

* this fix brings the ofShader #pragma include directive in line with the behaviour of a C preprocessor.
* include paths will now always be *relative to the file calling the include*
* tested on Windows (VS2012), OS X

Signed-off-by: tgfrerer <tim@poniesandlight.co.uk>
update changelog

* adds a short description of the bugfix for increased karma

Signed-off-by: tgfrerer <tim@poniesandlight.co.uk>
fix quirk in how absolute/relative paths are resolved in ofShader includes

Signed-off-by: tgfrerer <tim@poniesandlight.co.uk>
Merge pull request #2784 from tgfrerer/fix-ofShader-include-paths

fix ofShader include path behaviour