Window: Fix contents not being drawn if no windowskin is set
Bitmap#blt: Clamp source rect to source bitmap bounds

RGSS allows the source rectangle in both `blt` and
`stretch_blt` to lie outside the source bitmap bounds
(treating the missing data as (0, 0, 0, 0)) and to be
inverted (in which case the blitted image is also inverted).

This commit only hanldes a corner case that
arises in the game "Last Scenario"; emulating the full
RGSS behavior is however desirable.