Graphical corruption with DMA fills

[ccawley2011]

An issue I encountered when migrating ArcEm to the newer toolchain is that using dmaFillHalfWords to clear the main screen ends up corrupting the UI on the touch screen. From what I can tell, it relates to the fact that dmaStartFill16 in calico performs two separate 16-bit writes for REG_DMAxCNT_L and REG_DMAxCNT_H rather than a single 32-bit write like in previous versions of libnds, but I don't know why that would cause a problem.