go.*,gpu: don't (badly) re-implement Go module versioning

Changes to the gioui.org/shader module are generally breaking changes,
which means that a particular version of gioui.org must be build with a
particular version of gioui.org/shader. Until now, the gpu package
checked the module version against an expected version and would fail at
runtime if there's a mismatch.

This change replaces all that complexity with a simple procedural
change: bump the module major version of gioui.org/shader at each such
incompatible change. It doesn't matter that we'll eventually reach
gioui.org/v1234/shader; the module is internal and won't break clients.

Signed-off-by: Elias Naur <mail@eliasnaur.com>

go.mod

@@ -10,5 +10,5 @@ require (
 
 require (
 	gioui.org/cpu v0.0.0-20210817075930-8d6a761490d2
-	gioui.org/shader v0.0.0-20210821080300-98542fa6d725
+	gioui.org/shader v1.0.0
 )