Implement texture unit setter and getter

examples/learnopengl.com/01-getting-started/04-textures/main.cr

@@ -162,10 +162,10 @@ while LibGLFW.window_should_close(window).false?
   context.clear(:color)
 
   # bind textures on corresponding texture units
-  LibGL.active_texture(LibGL::TextureUnit::Texture0)
-  texture_1.bind(:texture_2d)
-  LibGL.active_texture(LibGL::TextureUnit::Texture1)
-  texture_2.bind(:texture_2d)
+  context.textures.unit = 0
+  texture_1.bind(texture_2d)
+  context.textures.unit = 1
+  texture_2.bind(texture_2d)
 
   # render container
   our_shader.use

spec/gloop/textures_spec.cr

@@ -97,6 +97,12 @@ Spectator.describe Gloop::Textures do
       expect(binding.target).to eq(Gloop::Texture::Target::Texture2D)
     end
   end
+
+  describe "#unit=" do
+    it "sets the active texture unit" do
+      expect { textures.unit = 5 }.to change(&.unit).to(5)
+    end
+  end
 end
 
 Spectator.describe Gloop::Context do

src/gloop/textures.cr

@@ -1,11 +1,13 @@
+require "./contextual"
+require "./parameters"
 require "./texture/bind_target"
 require "./texture/target"
-require "./contextual"
 
 module Gloop
   # Reference to all texture binding targets for a context.
   struct Textures
     include Contextual
+    include Parameters
 
     # Defines a method that returns a texture binding target for the specified target.
     #
@@ -59,6 +61,26 @@ module Gloop
     def [](target : Symbol) : Texture::BindTarget
       self[Texture::Target.new(target)]
     end
+
+    # Retrieves the active texture unit.
+    #
+    # - OpenGL function: `glGetIntegerv`
+    # - OpenGL enum: `GL_ACTIVE_TEXTURE`
+    # - OpenGL version: 2.0
+    @[GLFunction("glGetIntegerv", enum: "GL_ACTIVE_TEXTURE", version: "2.0")]
+    parameter(ActiveTexture, unit) do |value|
+      value - LibGL::TextureUnit::Texture0.value
+    end
+
+    # Sets the active texture unit.
+    #
+    # - OpenGL function: `glActiveTexture`
+    # - OpenGL version: 2.0
+    @[GLFunction("glActiveTexture", version: "2.0")]
+    def unit=(unit)
+      value = LibGL::TextureUnit::Texture0.value + unit
+      gl.active_texture(LibGL::TextureUnit.new(value))
+    end
   end
 
   struct Context