TGA "CanReadFile" is too lax and should be improved

[mwestphal]

Describe the bug
vtkTGAReader has a "CanReadFile" method to determine if a stream is a potential TGA file, but any random file can a false positive

To Reproduce
Steps to reproduce the behavior:

1. Download this file autzen.zip
2. Open the file using cat autzen.pts | f3d - --no-config --no-render --verbose
3. TGA reader is used, it should not even considered the file readable

Expected behavior
PTS reader being used, because TGA should reject the file

System Information:

• OS: Any (but linux for piping)
• GPU and GPU driver: Any

F3D Information
Paste the content of f3d --version or f3d-console.exe --version on Windows: master

Additional context
http://www.paulbourke.net/dataformats/tga/

A mentionned by @Meakk , it should be possible to check bitsperpixel. Should be 24 if we support RGB only, or at least a multiple of 8.

Fix must be in VTK.

[knobby-tires]

Opened an upstream fix for this in VTK: https://gitlab.kitware.com/vtk/vtk/-/merge_requests/13011