Fix multisource dtype bugs

[willdunklin]

Made sure that _addSourceToTile's sourceTile is being cast to the proper dtype for the tilesource. Previously all sourceTiles defaulted to float32 which caused certain issues with compositing (and contributed to the bug below).

Removed self.dtype references from _mergeTiles in lieu of the appropriate types recovered from the sourceTile changes. Using self.dtype here was causing issues for edge cases with alpha compositing. In TileSource.dtype, the _dtype value is set to 'check' temporarily to let the code sample a tile with getRegion (which typically needs a temp value for metadata checks); however, with MultiFileTileSource, the dtype was needed for getRegion sampling in a meaningful capacity (here with the fullAlphaValue calls) so a dtype = 'check' caused certain edge cases to crash.