gpu/internal/opengl: support sRGB emulation for desktop OpenGL

Desktop OpenGL implements a GL_FRAMEBUFFER_SRGB setting; query that instead
of the frambuffer color encoding.

With this change it is no longer necessary to enable FRAMEBUFFER_SRGB
in the macOS setup; remove it.

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

internal/gl/gl_windows.go

@@ -70,6 +70,7 @@ var (
 	_glGetString                          = LibGLESv2.NewProc("glGetString")
 	_glGetUniformLocation                 = LibGLESv2.NewProc("glGetUniformLocation")
 	_glInvalidateFramebuffer              = LibGLESv2.NewProc("glInvalidateFramebuffer")
+	_glIsEnabled                          = LibGLESv2.NewProc("glIsEnabled")
 	_glLinkProgram                        = LibGLESv2.NewProc("glLinkProgram")
 	_glPixelStorei                        = LibGLESv2.NewProc("glPixelStorei")
 	_glReadPixels                         = LibGLESv2.NewProc("glReadPixels")
@@ -347,6 +348,10 @@ func (c *Functions) InvalidateFramebuffer(target, attachment Enum) {
 	}
 	syscall.Syscall(addr, 3, uintptr(target), 1, uintptr(unsafe.Pointer(&attachment)))
 }
+func (f *Functions) IsEnabled(cap Enum) bool {
+	u, _, _ := syscall.Syscall(_glIsEnabled.Addr(), 1, uintptr(cap), 0, 0)
+	return u == TRUE
+}
 func (c *Functions) LinkProgram(p Program) {
 	syscall.Syscall(_glLinkProgram.Addr(), 1, uintptr(p.V), 0, 0)
 }