gpu,app,internal/glimpl: update GL backend for the compute renderer

Modern graphics APIs have immutable objects, with mutable data. For example,
a texture's dimensions are immutable, while the texture contents is not.
Change the GPU API abstraction to match.

Clearing a Texture is convenient to do with a plain []byte. Generalize
Texture.Upload to take a plain byte slice and introduce a helper function for
uploading *image.RGBA data.

Add TextureFormatRGBA8 a format for the linear RGB colorspace.

Add OpenGL ES 3.1 functions for compute programs.

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

gpu/gpu.go

@@ -479,7 +479,7 @@ func (r *renderer) texHandle(cache *resourceCache, data imageOpData) backend.Tex
 	if err != nil {
 		panic(err)
 	}
-	handle.Upload(data.src)
+	backend.UploadImage(handle, image.Pt(0, 0), data.src)
 	tex.tex = handle
 	return tex.tex
 }