gpu/internal/opengl: save and restore GL state only for shared contexts

Most Gio programs have exlusive access to their OpenGL contexts, where
saving and restoring of the GL state is not required.

This change applies to all OpenGL platforms, but matters most for WASM
where syscalls are slow.

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

gpu/internal/driver/api.go

@@ -50,6 +50,9 @@ type OpenGL struct {
 	// empty for all other platforms; an OpenGL context is assumed current when
 	// calling NewDevice.
 	Context gl.Context
+	// Shared instructs users of the context to restore the GL state after
+	// use.
+	Shared bool
 }
 
 type Direct3D11 struct {

gpu/internal/opengl/opengl.go

@@ -23,6 +23,7 @@ type Backend struct {
 	glstate    glState
 	state      state
 	savedState glState
+	sharedCtx  bool
 
 	glver [2]int
 	gles  bool
@@ -201,6 +202,7 @@ func newOpenGLDevice(api driver.OpenGL) (driver.Device, error) {
 		floatTriple: floatTriple,
 		alphaTriple: alphaTripleFor(ver),
 		srgbaTriple: srgbaTriple,
+		sharedCtx:   api.Shared,
 	}
 	b.feats.BottomLeftOrigin = true
 	if srgbErr == nil {
@@ -216,13 +218,20 @@ func newOpenGLDevice(api driver.OpenGL) (driver.Device, error) {
 		b.feats.Features |= driver.FeatureTimers
 	}
 	b.feats.MaxTextureSize = f.GetInteger(gl.MAX_TEXTURE_SIZE)
+	if !b.sharedCtx {
+		// We have exclusive access to the context, so query the GL state once
+		// instead of at each frame.
+		b.glstate = b.queryState()
+	}
 	return b, nil
 }
 
 func (b *Backend) BeginFrame(target driver.RenderTarget, clear bool, viewport image.Point) driver.Texture {
 	b.clear = clear
-	b.glstate = b.queryState()
-	b.savedState = b.glstate
+	if b.sharedCtx {
+		b.glstate = b.queryState()
+		b.savedState = b.glstate
+	}
 	b.state = state{}
 	var renderFBO gl.Framebuffer
 	if target != nil {
@@ -286,7 +295,9 @@ func (b *Backend) EndFrame() {
 		}
 		b.sRGBFBO.Blit()
 	}
-	b.restoreState(b.savedState)
+	if b.sharedCtx {
+		b.restoreState(b.savedState)
+	}
 }
 
 func (b *Backend) queryState() glState {