app,gpu,internal/egl: lock OS thread when making OpenGL context current

OpenGL stores the current context in thread-local memory, but commit
4f5baa9a51 removed a runtime.LockOSThread from app.Window that ensured
the goroutine that drives the context stays on the operating thread that
has the context current. This change restores the thread lock.

As a bonus, this change makes the OpenGL contexts responsible for locking
the thread at MakeCurrent, thereby removing LockOSThread calls from GPU
backend-agnostic code.

Fixes: https://todo.sr.ht/~eliasnaur/gio/334
Signed-off-by: Elias Naur <mail@eliasnaur.com>

gpu/headless/driver_test.go

@@ -165,7 +165,6 @@ func newDriver(t *testing.T) driver.Device {
 	if err != nil {
 		t.Skipf("no context available: %v", err)
 	}
-	runtime.LockOSThread()
 	if err := ctx.MakeCurrent(); err != nil {
 		t.Fatal(err)
 	}

gpu/headless/headless.go

@@ -8,7 +8,6 @@ import (
 	"errors"
 	"image"
 	"image/color"
-	"runtime"
 
 	"gioui.org/gpu"
 	"gioui.org/gpu/internal/driver"
@@ -146,8 +145,6 @@ func (w *Window) Screenshot(img *image.RGBA) error {
 func contextDo(ctx context, f func() error) error {
 	errCh := make(chan error)
 	go func() {
-		runtime.LockOSThread()
-		defer runtime.UnlockOSThread()
 		if err := ctx.MakeCurrent(); err != nil {
 			errCh <- err
 			return