End portal/end gateway starfield pixels are stretched according to window size

The bug

The pixels seen in end gateway and end portal blocks are stretched to the same shape of the game window. For end portals, this has been the case since 16w40a - all previous versions containing the block were not affected.

How to reproduce

1. View an end portal or end gateway
2. Look closely at the floating pixels within
3. Optionally, resize your window to some funny sizes

Expected results

All of the pixels in end portals would be perfect squares.

Actual results

Unless your game window is square, the pixels will be a rectangle (a mathematically similar rectangle to that of the game window).

How to fix

Have the end portal effect be truncated according to window size, rather than stretched.