Mali 450 precision issue: how to detect odd and even columns

Hi,

I'm working on an algorithm where I need to detect if the current `gl_FragCoord` is an even or odd row/column. A typical implementation would look something like `if (mod(gl_FragCoord.x, 2.0) < 1.0) { ... } else { ... }`.  After running into issues with this approach a quick google search pointed me to quite some good information:

• Benchmarking floating-point precision in mobile GPUs
• Mobile GPU Floating Point Accuracy Variances
• Float precision and Mobile GPUs
• Shader data types and precision (Unity 3D)
• Forum post: Suggestions for testing Android GLSL against Mali GPUs
• At Home on the Range - Why Floating Point Formats Matter in Graphics

Especially this forum post is pretty much identical to the issue I'm looking into.

I've created this repository to experiment with this issue and to solve it. I'm using a MiBox MDZ-16-AB.  This repository is created for Android Studio. It will setup a basic custom `GLSurfaceView` that uses the `TestGlView` that instantiates the `TestGlRenderer` class. In `TestGlRenderer` I create a simple filter shader that applies the `if (mod(..))` logic to draw different colors for odd/even columns.  I create a FBO with a size of 3840 x 2160 to detect floating point precision issues. When rendering into a FBO with a color attachment of a texture with a size of 1920 x 1080 the issues is a lot less.

In the image below (3840 x 2160) you can clearly see the issue. It should show vertical red and black lines from left to right. 

When rendering using a 1920 x 1080 things get a little bit better, but still not 100% correct.

In `TestGlRenderer` I create an instance of `GlRenderToTexture` which is just a thin wrapper that create a FBO with one texture attachment. In `onSurfaceCreated()` of the `TestGlRenderer` class I create an instance of `GlRenderToTexture`. In the code I've added (a commented) version which creates either a 1920x1080 or 3840x2160 FBO. 

Now I'm curious what would be a workaround or solution to be able to distinguish between odd and even rows/columns?

Thanks