Alpha transparency in mask

[kfrajer]

This might be related to #453

Reported in Processing forum here.

Tested using Win10 using processing-phone emulator as well on device. Images from emu.

Issue:

The following code produces a glitch image in the masked area when using the default renderer:

fullScreen();
img.mask(maskImg);
image(img, width/2, height/2);

This is not observed with the P2D renderer: