Splashscreen uses libjpeg-internal macros

As indicated in the libjpeg API documentation [1], the standard libjpeg headers are: jpeglib.h, jerror.h, jconfig.h, jmorecfg.h

splashscreen_jpeg.c violates this layering and uses the SIZEOF macro from the non-public header jinclude.h directly. This macro is a wrapper over sizeof. The code in splashscreen_jpeg.c already uses sizeof in multiple places so it seems like it should be okay to use it in some more.

One additional motivation of having this patch is that if/when we add system-libjpeg support, the header jinclude.h will become inaccessible.

[1] http://svn.code.sf.net/p/libjpeg-turbo/code/trunk/libjpeg.txt