tempblend can't be read back unlike other temp values in m32

Line 597 starts temp clipboard content vars inside source/duke3d/src/m32vars.cpp

Possibly when tempblend was added, it was never exposed as a RO variable. A line with Gv_NewVar("tempblend",(intptr_t)&tempblend, GAMEVAR_INTPTR|GAMEVAR_SYSTEM|GAMEVAR_READONLY); will remedy this.