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>

app/gl_ios.go

@@ -31,6 +31,7 @@ import "C"
 import (
 	"errors"
 	"fmt"
+	"runtime"
 
 	"gioui.org/gpu"
 	"gioui.org/internal/gl"
@@ -101,6 +102,9 @@ func (c *context) Present() error {
 }
 
 func (c *context) Lock() error {
+	// OpenGL contexts are implicit and thread-local. Lock the OS thread.
+	runtime.LockOSThread()
+
 	if C.gio_makeCurrent(c.ctx) == 0 {
 		return errors.New("[EAGLContext setCurrentContext] failed")
 	}