Implements a new SH light grid that runs alongside the legacy Q3 light grid,
storing 9 RGB L2 spherical harmonic coefficients per grid point for accurate
directional lighting of dynamic objects from all angles.
BSP format: v47 with 19-lump header (160 bytes) when -sh is used, v46 with
17-lump header (144 bytes) otherwise. SH data stored in LUMP_LIGHTGRID_SH
(index 18) with a header containing grid bounds/size/mins followed by the
coefficient array. Stock Q3 engines read v46 lumps unchanged.
New CLI flags: -sh (enable), -gridscalesh (density multiplier, default 2x),
-gridsh (explicit cell size). SH grid receives bounced light with -bouncegrid.
Also adds libjpeg-turbo as a proper build dependency with its own vcxproj,
fixing the previous external engine path requirement.
