Add checkerboard test pattern

[wsygog]

[fsphil]

Hi, thanks for the pull request. Just a few comments:

• Your commit is removing indentations throughout hacktv.c and test.c. I keep tab indentations on empty lines and your editor is removing them. Can you limit the changes to just the new code.
• Pass the sub string to av_test_open() rather than the integer. Make the decision on which pattern to render within av_test_open() instead of hacktv.c.
• Would it be better to have the pattern fill the screen?

[wsygog]

Oops! Will fix the indentations when I can.

Good point with passing sub instead of an arbitrary number.

I'll see how full screen looks when I can. Only thing is, my only analog TV is pretty bad, so I'm afraid what is full screen for me isn't for everyone else haha

[wsygog]

@fsphil With what you said, i think sections inside av_test_open() for Overlay the logo and Generate a basic test pattern should be done only if sub == NULL or strncmp("test") is successful. Likewise for sections Draw checkerboard. What do you think? Also i can't test this since I will be away from my television for a week.