Fix most of staticcheck errors

[dim13]

• fix most issues reported by staticcheck
• fix issues reported by go vet
• update go version to 1.21
• apply gofmt -s

[aarzilli]

Many of the files you are changing are autogenerated.

[jezek]

@dim13 As @aarzilli says, many files are autogenerated. Here is the list (obtained with command grep -rl '// This file is automatically generated from'|grep -v xgbgen|sort):

bigreq/bigreq.go
composite/composite.go
damage/damage.go
dpms/dpms.go
dri2/dri2.go
ge/ge.go
glx/glx.go
randr/randr.go
record/record.go
render/render.go
res/res.go
screensaver/screensaver.go
shape/shape.go
shm/shm.go
xcmisc/xcmisc.go
xevie/xevie.go
xf86dri/xf86dri.go
xf86vidmode/xf86vidmode.go
xfixes/xfixes.go
xinerama/xinerama.go
xprint/xprint.go
xproto/xproto.go
xselinux/xselinux.go
xtest/xtest.go
xvmc/xvmc.go
xv/xv.go

Corrections should be made in the generator (xgbgen) and then files regenerated as last commit.

[dim13]

Yes, you're absolutely right, I've overseen, that many files are generated. This PR as is now, makes no sense then. I would rather close it, and come back with more thought-through solution later.

[dim13]

Rework needed