do not add alpha band if it exists (#176)

terracotta/drivers/raster_base.py

@@ -393,8 +393,11 @@ def _get_resampling_enum(method: str) -> Any:
 
     @staticmethod
     def _has_alpha_band(src: 'DatasetReader') -> bool:
-        from rasterio.enums import MaskFlags
-        return any([MaskFlags.per_dataset in flags for flags in src.mask_flag_enums])
+        from rasterio.enums import MaskFlags, ColorInterp
+        return (
+            any([MaskFlags.alpha in flags for flags in src.mask_flag_enums])
+            or ColorInterp.alpha in src.colorinterp
+        )
 
     @classmethod
     @trace('get_raster_tile')
@@ -478,8 +481,9 @@ def _get_raster_tile(cls, path: str, *,
             # construct VRT
             vrt = es.enter_context(
                 WarpedVRT(
-                    src, crs=cls._TARGET_CRS, resampling=reproject_enum, add_alpha=True,
-                    transform=vrt_transform, width=vrt_width, height=vrt_height
+                    src, crs=cls._TARGET_CRS, resampling=reproject_enum,
+                    transform=vrt_transform, width=vrt_width, height=vrt_height,
+                    add_alpha=not cls._has_alpha_band(src)
                 )
             )
 
@@ -489,9 +493,11 @@ def _get_raster_tile(cls, path: str, *,
                 tile_data = vrt.read(
                     1, resampling=resampling_enum, window=out_window, out_shape=tile_size
                 )
-                # read alpha mask
-                mask_idx = src.count + 1
+
+                # assemble alpha mask
+                mask_idx = vrt.count
                 mask = vrt.read(mask_idx, window=out_window, out_shape=tile_size) == 0
+
                 if src.nodata is not None:
                     mask |= tile_data == src.nodata