gpu,gpu/gl: introduce Backend

A recent change made the OpenGL functions an interface of the functions
required for the implementation of GPU, a renderer for Gio operations.
That allowed for running Gio on external systems where OpenGL is
available.

However, to allow for non-OpenGL flavored backends such as Vulkan,
Metal and Direct3D, this change introduces Backend for the high-level
operations required by GPU. This change also adds a concrete backend
to package gl.

Type Backend is a first cut heavily based on OpenGL. Future changes will add
more backends, where the Backend interface quite possibly will need refinement.

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

app/internal/glimpl/gl_windows.go

@@ -11,6 +11,7 @@ import (
 	"golang.org/x/sys/windows"
 
 	"gioui.org/gpu/gl"
+	gunsafe "gioui.org/internal/unsafe"
 )
 
 var (
@@ -290,7 +291,7 @@ func (c *Functions) GetShaderInfoLog(s gl.Shader) string {
 }
 func (c *Functions) GetString(pname gl.Enum) string {
 	s, _, _ := syscall.Syscall(_glGetString.Addr(), 1, uintptr(pname), 0, 0)
-	return gl.GoString(gl.SliceOf(s))
+	return gunsafe.GoString(gunsafe.SliceOf(s))
 }
 func (c *Functions) GetUniformLocation(p gl.Program, name string) gl.Uniform {
 	cname := cString(name)