gpu: re-align coverUniforms struct

Direct3D requires GPU vertex attribute structs sizes be a multiple
16. A cleanup commit removed an unusued field, and broke that
assumption.

Fixes: https://todo.sr.ht/~eliasnaur/gio/422
Signed-off-by: Elias Naur <mail@eliasnaur.com>

gpu/path.go

@@ -58,6 +58,7 @@ type coverUniforms struct {
 	uvCoverTransform [4]float32
 	uvTransformR1    [4]float32
 	uvTransformR2    [4]float32
+	_                float32
 }
 
 type stenciler struct {