shaders@glsl-max-varyings: reduce size of pixels

With a pixel size of 2x2 size the drawing area is 1026x1026, added to that window decorations, and possible offsets from the desktop environment the window might not fit on the screen so that the desktop manager might scale the window to fit the screen. Running the piglit without the -fbo flag will then lead to reading back the output from the wrong location.

Reduce the size ofthe pixels to 1x1 so that we end up with a drawing area of 514, which is more likely to fit on the screen.