gpu: remove Backend.NilTexture

It serves no purpose other than paranoia. Perhaps buggy drivers exists that
require unused texture slots cleared before drawing to a texture, but if so the
workaround belongs in the GL backend.

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

gpu/backend.go

@@ -20,7 +20,6 @@ type Backend interface {
 	IsTimeContinuous() bool
 	NewTexture(format TextureFormat, width, height int, minFilter, magFilter TextureFilter) Texture
 	DefaultFramebuffer() Framebuffer
-	NilTexture() Texture
 	NewFramebuffer() Framebuffer
 	NewImmutableBuffer(typ BufferType, data []byte) Buffer
 	NewBuffer(typ BufferType, size int) Buffer

gpu/gl/backend.go

@@ -167,10 +167,6 @@ func (b *Backend) NewFramebuffer() gpu.Framebuffer {
 	return &gpuFramebuffer{funcs: b.funcs, obj: fb}
 }
 
-func (b *Backend) NilTexture() gpu.Texture {
-	return &gpuTexture{backend: b}
-}
-
 func (b *Backend) DefaultFramebuffer() gpu.Framebuffer {
 	return b.defFBO
 }

gpu/path.go

@@ -279,7 +279,6 @@ func (p *pather) stencilPath(bounds image.Rectangle, offset f32.Point, uv image.
 }
 
 func (s *stenciler) beginIntersect(sizes []image.Point) {
-	s.ctx.NilTexture().Bind(1)
 	s.ctx.BlendFunc(BlendFactorDstColor, BlendFactorZero)
 	// 8 bit coverage is enough, but OpenGL ES only supports single channel
 	// floating point formats. Replace with GL_RGB+GL_UNSIGNED_BYTE if
@@ -304,7 +303,6 @@ func (s *stenciler) cover(idx int) stencilFBO {
 }
 
 func (s *stenciler) begin(sizes []image.Point) {
-	s.ctx.NilTexture().Bind(1)
 	s.ctx.BlendFunc(BlendFactorOne, BlendFactorOne)
 	s.fbos.resize(s.ctx, sizes)
 	s.ctx.ClearColor(0.0, 0.0, 0.0, 0.0)