Fix #192: tiff files (and others) appear blurry when 1:1 view

#192

Up to commit 808d855 alpha-channel files at scale 1:1 always had
GdkInterpType set to GDK_INTERP_NEAREST - the lowest quality. As there
was no scaling, this was ok.
Thereafter scaling was set to that selected by the user, resulting in
problems when GDK_INTERP_HYPER was selected.
This commit reverts to the previous method.

src/renderer-tiles.c

@@ -1270,7 +1270,7 @@ static void rt_tile_get_region(gboolean has_alpha,
 					 offset_x,
 					 offset_y,
 					 scale_x, scale_y,
-					 interp_type,
+					 (scale_x == 1.0 && scale_y == 1.0) ? GDK_INTERP_NEAREST : interp_type,
 					 255, check_x, check_y,
 					 PR_ALPHA_CHECK_SIZE,
 					 ((options->image.alpha_color_1.red << 8 & 0x00FF0000) +