Improve preprocessor to allow for define value injection

[mvaligursky]

Improvement to Preprocessor to allow for defines starting with __INJECT_ to be injected into the processed text. This will be used during further shader refactoring, in order to remove manual shader code processing.

for example source code with defines:

        #define __INJECT_COUNT 2
        #define __INJECT_DECODE_SKYDOME decodeGamma

        for (int i = 0; i < __INJECT_COUNT; i++) {
            vec3 data = __INJECT_DECODE_SKYDOME(src);
        }

will generate code:

        for (int i = 0; i < 2; i++) {
            vec3 data = decodeGamma(src);
        }