fix Bitmap's object to string conversion

Calling #to_s might not return a string (it should though).

The RGSS also calls `rb_obj_as_string`:
```ruby
class A
  B = 'Test'
  def to_s; B; end
end
s = Sprite.new
s.bitmap = Bitmap.new(Graphics.width, Graphics.height)
p A::B.tainted?  # => false
a = A.new
a.taint  # a is tainted, not A::B!
s.bitmap.draw_text(s.bitmap.rect, a)   # rb_obj_as_string() taints the return value of A#to_s (A::B)
p A::B.tainted?  # => true
rgss_stop
```
Don't worry about tainting, I just wanted to prove it (for some reason). More importantly is that RGSS classes don't check if the string contains `'\0'`. For example, drawing `"Te\0st"` doesn't raise an error like it should (see “poison null byte”) and only draws `Te`. Should mkxp?