app,app/internal/gpu: split render loop from GPU

The policy of rendering on a separate goroutine is separate from
the actual rendering. Reflect that by introducing the RenderLoop
type for driving a GPU from a separate goroutine.

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

app/internal/gpu/context.go

@@ -32,9 +32,9 @@ type textureTriple struct {
 	typ            gl.Enum
 }
 
-func newContext(glctx gl.Context) (*context, error) {
+func newContext(glctx *gl.Functions) (*context, error) {
 	ctx := &context{
-		Functions: glctx.Functions(),
+		Functions: glctx,
 	}
 	exts := strings.Split(ctx.GetString(gl.EXTENSIONS), " ")
 	glVer := ctx.GetString(gl.VERSION)