This change refactors the setting of the compilation mode, so that
instead of using a 'gOfflineCompiler' bool together with a
'gOfflineCompilerOutputType' enum, a single 'gCompilationMode' enum
is used. The default value for gCompilationMode is 'kOnline', which
is equivalent to the previous defaulting of gOfflineCompiler to false.
In addition, it refactors the setting of the compilation cache mode,
so that instead of the 'gForceSpirVCache' and 'gForceSpirVGenerate'
bools, a single 'gCompilationCacheMode' enum is used. The default
value for gCompilationCacheMode is 'kCacheModeCompileIfAbsent', which
is equivalent to the previous defaulting of both booleans to false.
