Uses a signed char instead of a char for opcode stack offset. On some platforms like ARM, char is unsigned by default

This fixes #311 where a memory corruption that occurred in ARM was caused because of insufficient ScriptVM stack size. The stack size was incorrectly calculated because the stack offset was using unsigned values instead of signed in ARM.

code/script/scriptcompiler.cpp

@@ -77,7 +77,7 @@ unsigned char ScriptCompiler::PrevOpcode()
     return prev_opcodes[prev_opcode_pos].opcode;
 }
 
-char ScriptCompiler::PrevVarStackOffset()
+signed char ScriptCompiler::PrevVarStackOffset()
 {
     return prev_opcodes[prev_opcode_pos].VarStackOffset;
 }