gpu/internal/opengl: ensure Backend.queryState is idempotent

Backend.queryState sets GL_ACTIVE_TEXTURE to access texture bindings; this change
makes sure GL_ACTIVE_TEXTURE is restored.

Also, make sure GL state changes made in Backend.restoreState is reflected
in Backend.glstate. Backend.glstate is not used after restoreState, so this
is just for clarity.

This is an alternative to https://github.com/gioui/gio/pull/77.

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

gpu/internal/opengl/opengl.go

@@ -339,10 +339,12 @@ func (b *Backend) queryState() glState {
 			s.storeBufs[i] = gl.Buffer(b.funcs.GetBindingi(gl.SHADER_STORAGE_BUFFER_BINDING, i))
 		}
 	}
+	active := s.texUnits.active
 	for i := range s.texUnits.binds {
 		s.activeTexture(b.funcs, gl.TEXTURE0+gl.Enum(i))
 		s.texUnits.binds[i] = gl.Texture(b.funcs.GetBinding(gl.TEXTURE_BINDING_2D))
 	}
+	s.activeTexture(b.funcs, active)
 	for i := range s.vertAttribs {
 		a := &s.vertAttribs[i]
 		a.enabled = b.funcs.GetVertexAttrib(i, gl.VERTEX_ATTRIB_ARRAY_ENABLED) != gl.FALSE
@@ -357,7 +359,7 @@ func (b *Backend) queryState() glState {
 }
 
 func (b *Backend) restoreState(dst glState) {
-	src := b.glstate
+	src := &b.glstate
 	f := b.funcs
 	for i, unit := range dst.texUnits.binds {
 		src.bindTexture(f, i, unit)