Fix text rendering with 8bpp-simple blitter

First member of string_colourremap was never assigned
Ensure that it is set to 0

src/gfx.cpp

@@ -498,6 +498,7 @@ void GfxBlitterCtx::SetColourRemap(TextColour colour)
 	bool raw_colour = (colour & TC_IS_PALETTE_COLOUR) != 0;
 	colour &= ~(TC_NO_SHADE | TC_IS_PALETTE_COLOUR | TC_FORCED);
 
+	this->string_colourremap[0] = 0;
 	this->string_colourremap[1] = raw_colour ? (byte)colour : _string_colourmap[colour];
 	this->string_colourremap[2] = no_shade ? 0 : 1;
 	this->colour_remap_ptr = this->string_colourremap;