iidx, linux: cache lstat result for movie thumbnails (#474)

## Link to GitHub Issue, if one exists
https://codeberg.org/nixac/spicetools/issues/4

## Description of change
Inspired by the Linux issue above in the fork, cache lstat calls for
thumbnail. IIDX 32 and 33 make a call for this every frame which can
cause significant overhead on Linux.

This is being enabled for all versions and not just the Linux variant of
the executables since it provides some marginal performance gain on
Windows.

## Testing
Tested IIDX 32/33, with the option on/off, and with AVS verbose logging
on/off.

Tested with ifs_layeredfs hook as well just to verify functionality.

src/spice2x/launcher/options.h

@@ -196,6 +196,7 @@ namespace launcher {
             DisableSignalHandling,
             DisableDebugHooks,
             DisableAvsVfsDriveMountRedirection,
+            DisableAvsCache,
             OutputPEB,
             DumpSystemInfo,
             QKSArgs,