gpu/backend: remove clear color and depth state

Specifying the clear color and depth at the time of clearing is
less error prone and a better for modern GPU APIs. As a bonus, we
can get rid of the BufferAttachment type.

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

gpu/backend/backend.go

@@ -27,9 +27,8 @@ type Device interface {
 	NewInputLayout(vertexShader ShaderSources, layout []InputDesc) (InputLayout, error)
 
 	DepthFunc(f DepthFunc)
-	ClearColor(r, g, b, a float32)
 	ClearDepth(d float32)
-	Clear(buffers BufferAttachments)
+	Clear(r, g, b, a float32)
 	Viewport(x, y, width, height int)
 	DrawArrays(mode DrawMode, off, count int)
 	DrawElements(mode DrawMode, off, count int)
@@ -110,8 +109,6 @@ type BlendFactor uint8
 
 type DrawMode uint8
 
-type BufferAttachments uint
-
 type TextureFilter uint8
 type TextureFormat uint8
 
@@ -157,11 +154,6 @@ type Texture interface {
 	Release()
 }
 
-const (
-	BufferAttachmentColor BufferAttachments = 1 << iota
-	BufferAttachmentDepth
-)
-
 const (
 	DepthFuncGreater DepthFunc = iota
 )