Only retrieve modification time once and remove warning if failed
Only retrieve modification time once and remove warning if failed

This change removes the warning message if retrieving the soundfont file
modification time fails in the sample cache loader. The warning made sense
while we didn't have mtime support on all platforms, but after switching
to GLibs g_stat, it's no longer needed.

To reduce the number of calls to fluid_get_file_modification_time and also
get rid of a possible race condition, getting the mtime has been moved to
fluid_samplecache_load.

This change also fixes #483 because it removes the bogus warning messages
if a soundfont was loaded from memory by abusing the filename to store a
pointer.

FluidSynth/fluidsynth,FluidSynth/fluidsynth,FluidSynth/fluidsynth,FluidSynth/fluidsynth