[truetype] Reload context after re-executing `prep`
When a different hinting mode from the current is selected, the prep
must be re-executed with the new mode. After this happens the context
must be re-loaded in preparation for the glyph program to be run.
- truetype/ttgload.c (tt_loader_init): add call to TT_Load_Context
Fixes: #1104 (closed)