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>

app/internal/window/d3d11_windows.go

@@ -94,8 +94,12 @@ func (c *d3d11Context) Release() {
 	if c.fbo != nil {
 		c.fbo.Release()
 	}
-	c.swchain.Release()
-	c.Device.Release()
+	if c.swchain != nil {
+		c.swchain.Release()
+	}
+	if c.Device != nil {
+		c.Device.Release()
+	}
 	c.fbo = nil
 	c.swchain = nil
 	c.Device = nil