[babl] type-u8: fix u8 <> double conversions for chroma type

[Øyvind Kolås <ok src gnome org>]

commit 00799b3c93a9e63f40412c733ad13e452b2048a7
Author: Thomas Manni <thomas manni free fr>
Date:   Sun Mar 4 10:17:07 2018 +0100

    type-u8: fix u8 <> double conversions for chroma type

 babl/base/type-u8.c |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)
---
diff --git a/babl/base/type-u8.c b/babl/base/type-u8.c
index 8eeaf65..d17590d 100644
--- a/babl/base/type-u8.c
+++ b/babl/base/type-u8.c
@@ -110,7 +110,7 @@ convert_u8_double_scaled (BablConversion *c,
 
 MAKE_CONVERSIONS (u8, 0.0, 1.0, 0x00, UINT8_MAX)
 MAKE_CONVERSIONS (u8_luma, 0.0, 1.0, 16, 235)
-MAKE_CONVERSIONS (u8_chroma, 0.0, 1.0, 16, 240)
+MAKE_CONVERSIONS (u8_chroma, -0.5, 0.5, 16, 240)
 
 void
 babl_base_type_u8 (void)