gpu/internal/opengl: handle switch from non-sRGB output to sRGB output

The opengl example has a screenshot functionality that renders to a
non-sRGB texture, whereas window rendering is to a sRGB capable EGL
surface. The opengl driver detects the switch from an sRGB capable
output to a non-sRGB capable output, but not the switch back. This
change releases the emulation framebuffer on the switch back.

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

gpu/internal/opengl/opengl.go

@@ -244,6 +244,9 @@ func (b *Backend) BeginFrame(target driver.RenderTarget, clear bool, viewport im
 				panic(err)
 			}
 			renderFBO = b.sRGBFBO.Framebuffer()
+		} else if b.sRGBFBO != nil {
+			b.sRGBFBO.Release()
+			b.sRGBFBO = nil
 		}
 	} else {
 		b.glstate.set(b.funcs, gl.FRAMEBUFFER_SRGB, true)