gpu,gpu/backend: implement generic backend.NewDevice

NewDevice creates a Device given an API, which is the necessary GPU
resources for a backend.

Convert gpu.New to take an API instead of a backend.Device directly.

In turn, this frees us to later unexport the backend package along with
the backend implementations (for now just gioui.org/gpu/gl for OpenGL).
It also allows programs that embed Gio (such as gioui.org/example/glfw)
to freely choose a backend, not just OpenGL.

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

gpu/api.go

@@ -0,0 +1,15 @@
+// SPDX-License-Identifier: Unlicense OR MIT
+
+package gpu
+
+import "gioui.org/gpu/backend"
+
+// An API carries the necessary GPU API specific resources to create a Device.
+// There is an API type for each supported GPU API such as OpenGL and Direct3D.
+type API = backend.API
+
+// OpenGL denotes the OpenGL or OpenGL ES API.
+type OpenGL = backend.OpenGL
+
+// Direct3D11 denotes the Direct3D API.
+type Direct3D11 = backend.Direct3D11