Added an option for High Quality image downsampling for OpenGL canvas

1) This option activates a custom shader, which filters the image
with a variation of trapezoidal filter using "one-but-nearest"
mipmap level. This gives quite decent results, although it might
be not perfect from the maths point of view.

2) The patch also sets the default texture size to 256. Otherwise
activation of mipmap generation will make the painting with smaller
brushes insanely slow. This happens because the mipmap is regenerated
for the whole texture every time we change a small portion of it.