Re-create test/data/bricks-gray.tga

We now convert it from the BMP file, not the PNG file:
$ convert bricks-gray.bmp -auto-orient bricks-gray.tga

The bricks-gray.png file has some gamma correction metadata that meant
that the previous TGA file (when converted to NIA) didn't match the
other bricks-gray.* files. After this commit, they all match:

$ grep bricks-gray test/nia-checksums-of-data.txt
c2bce675 test/data/bricks-gray.bmp
c2bce675 test/data/bricks-gray.gif
c2bce675 test/data/bricks-gray.no-ancillary.png
c2bce675 test/data/bricks-gray.png
c2bce675 test/data/bricks-gray.tga

Updates #67
diff --git a/test/data/bricks-gray.tga b/test/data/bricks-gray.tga
index 9cb0b24..a72dd47 100644
--- a/test/data/bricks-gray.tga
+++ b/test/data/bricks-gray.tga
Binary files differ
diff --git a/test/nia-checksums-of-data.txt b/test/nia-checksums-of-data.txt
index 555bca4..16ed6c3 100644
--- a/test/nia-checksums-of-data.txt
+++ b/test/nia-checksums-of-data.txt
@@ -35,7 +35,7 @@
 c2bce675 test/data/bricks-gray.gif
 c2bce675 test/data/bricks-gray.no-ancillary.png
 c2bce675 test/data/bricks-gray.png
-0319cb08 test/data/bricks-gray.tga
+c2bce675 test/data/bricks-gray.tga
 7aebfe6c test/data/bricks-nodither.bmp
 7aebfe6c test/data/bricks-nodither.gif
 7aebfe6c test/data/bricks-nodither.png