Make colour/2 a top-level function

This makes it more obvious that this function's effect is permanent
until the next invocation.

lib/mudbrick.ex

@@ -68,16 +68,11 @@ defmodule Mudbrick do
     |> ContentStream.add(ContentStream.Td, tx: x, ty: y)
   end
 
-  def text(context, text, opts \\ []) do
-    context =
-      case Keyword.get(opts, :colour) do
-        {r, g, b} ->
-          ContentStream.add(context, ContentStream.Rg, r: r, g: g, b: b)
-
-        _ ->
-          context
-      end
+  def colour(context, {r, g, b}) do
+    ContentStream.add(context, ContentStream.Rg, r: r, g: g, b: b)
+  end
 
+  def text(context, text, opts \\ []) do
     ContentStream.write_text(context, text, opts)
   end
 

test/mudbrick/font_test.exs

@@ -23,13 +23,11 @@ defmodule Mudbrick.FontTest do
       )
       |> font(:helvetica, size: 10)
       |> text("black and ")
-      |> text(
-        """
-        red
-        text\
-        """,
-        colour: {1.0, 0.0, 0.0}
-      )
+      |> colour({1.0, 0.0, 0.0})
+      |> text("""
+      red
+      text\
+      """)
 
     assert show(content_stream) =~
              """

test/mudbrick_test.exs

@@ -11,14 +11,13 @@ defmodule MudbrickTest do
            )
            |> text_position(200, 700)
            |> font(:bodoni, size: 14)
-           |> text("CO₂ ", colour: {1, 0, 0}, align: :right)
-           |> text(
-             """
-             is Carbon Dioxide
-             and HNO₃ is Nitric Acid
-             """,
-             colour: {0, 0, 0}
-           )
+           |> colour({1, 0, 0})
+           |> text("CO₂ ", align: :right)
+           |> colour({0, 0, 0})
+           |> text("""
+           is Carbon Dioxide
+           and HNO₃ is Nitric Acid
+           """)
            |> text("wide stuff", align: :right)
            |> text("wider stuff", align: :right)
            |> text("z", align: :right)