Polar compute shader recompilations triggered by floating point inaccuracy

When rx / ry are close to an integer multiple upscaling ratio (e.g. 2.0), the calculation of ceil(bw / iw) can easily bounce ±1, as a result of ceil rounding up values ever-so-slightly above 1.

This is very bad as it triggers shader recompilations, possibly frequently so as a result of benign and tiny changes in the source crop (e.g. as a consequence of aspect-ratio preserving). There needs to be a better solution to make this calculation more stable.