ShaderPreprocessor doesn't work on android

[richardeakin]

Not a fault of android, just that android doesn't use direct file access like other platforms. Problem is that ShaderPreprocessor tries to directly access files with ifstream( fullFilePath ), instead of getting routed through the android assets mechanisms. I propose we add a std::function based 'filepath resolver' callback that returns either a ci::DataSourceRef or ci::IStreamRef, providing a workable default that uses ci::loadFile but also allows users to override when needed.

[richardeakin]

include path handler added in #1721, GlslProg (or something else) should add a custom handler to load strings via android's asset file system stuff.