Bug 39457

Shared-mime-info prefers a Windows cursor (image/x-win-bitmap) over an
uncompressed TGA (image/x-tga).

Windows cursor:
<magic priority="50">
  <match type="string" value="\0\0\2\0" offset="0">
    <match type="string" value="\0" offset="5"/>
  </match>
</magic>

TGA image:
<magic priority="10">
  <match type="string" value="\1\1" offset="1"/>
  <match type="string" value="\1\9" offset="1"/>
  <match type="string" value="\0\3" offset="1"/>
  <match type="string" value="\0\xa" offset="1"/>
  <match type="string" value="\0\xb" offset="1"/>
  <match type="string" value="\0\2" offset="1"/> <!-- Same magic as CUR files -->
</magic>

(Only the last match tag is important for _uncompressed_ TGA images.)

Unfortunately this breaks applications that depends on the correct MIME type
recognition from the file content. For example, geegie [1] is affected. Geeqie
uses pixbuf loader from gdk-pixbuf that is tricked into using an ICO loader
instead of a TGA loader in such a case.

Attached patch tries to address this issue. It utilizes the byte at the offset
16 (0x10).

In the TGA file format [2] this byte contains 'Pixel depth'. Common values are
8, 16, 24 and 32.

In the CUR file format [3] this byte is part of a DWORD that specifies a number
of bytes of the first cursor resource. It's the third byte in that DWORD
meaning it contains a value of bits 16-23 (little-endian). This byte should
always be 0 because cursor images are relatively small. (For example, among
Win 7 system cursors the highest value of this DWORD is 0x25a8.)

The patch adds check of the offset 16 only to the uncompressed TGA file magic
but you might consider adding it to all other TGA file patterns, or making CUR
file magic more strict.

[1] http://geeqie.sourceforge.net/
[2] http://www.dca.fee.unicamp.br/~martino/disciplinas/ea978/tgaffs.pdf
[3] http://www.digicamsoft.com/bmp/bmp.html