Fix gtk4-demo’s gears demo on OpenGL ES

Here is a command to reproduce this testcase:

GDK_DEBUG=gl-gles gtk4-demo --run gears

Without this patch, Mesa throws this compile error:

0:130(13): error: no matching function for call to `mod(error, float)'; candidates are:

This is caused by u_rotation - 90 being of type error since u_rotation is a float and it’s illegal to subtract it with an integer.