[rtextures] ImageDraw(): Don't try to blend images without alpha

[Not-Nik]

This PR enables ImageDraw() to work with all formats that are not compressed and don't have an alpha channel. This specifically allows loading cubemaps again, which before did not work for any layouts except CUBEMAP_LAYOUT_LINE_VERTICAL, possibly since 589d606000a61aaf9d7f14089a83a2bb30c3e714, because the function attempts to restructure cubemap images into a vertical line.