Testing Screenshots

Hello reader,

This week I updated and tested the screenshot ability of Vispy.

Normally, Vispy utilizes `read_pixel()` fuction which basically read pixels from a GL context. This function returns numpy array but somehow an upside-down array. So what I did was to flip that array which is extremely easy in Python and adding a test for it. The test was very simple too. We draw to top left a white square on a black screen. Then after reading pixels, we check whether the image is flipped or not by calculating the sum of the pixels in the corners. If the top left corner's pixel values are greater than 0 and the rest are equal to 0, it means we correctly took the screenshot.  You should know that I have never dealt with drawing something with GL functions before, that was my first.

Now I am making another PR with Luke Campagnola's -a vispydev- pure python PNG writer. This will help us to remove another image library dependency, like PIL. More updates will be coming after it has been merged.

See you next post!