Shader appears vertically

Retroarch 1.4.1 with glupen64 core and this shader:

\shaders\shaders_glsl\crt\shaders\dotmask.glsl

Aspect 4:3 - Core resolution set at 1600x1200

The shader appears vertically while it appears horizontally (correct) with other resolutions. Any ideas how to fix?

When you increase the core resolution, you also increase the framebuffer size and the shader gets applied to that framebuffer. The vertical orientation is sort of an optical illusion caused by the mask being applied at the increased res.