Fix tilemap UV rounding error

[grind086]

Objective

Fixes #22250

Solution

Using fract in the shader produces the wrong value if the input uv is outside the range 0 .. 1. Instead, subtract the already-calculated floor value to shift the uv into the correct range.

Testing

The visible seams are gone in my own testing, and a user on discord reported that this worked for them as well.

[alphadragon2]

I've tested this change locally on my machine and it does in fact fix the tilemap bleeding issue.