Fix gtk4-demo’s gears demo on OpenGL ES

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.