Make Mako preprocessing a Rake task

Per feedback from @cpennington. This way we'll only preprocess once,
even if the Sass compilation is invoked multiple times.

rakefiles/assets.rake

@@ -25,13 +25,6 @@ def preprocess_with_mako(filename)
     end
 end
 
-# Preprocess all static assets that have the .mako extension by
-# running them through the Mako templating engine. Right now we
-# just hardcode the asset filenames.
-def preprocess_assets
-    preprocess_with_mako("lms/static/sass/application.scss.mako")
-end
-
 def xmodule_cmd(watch=false, debug=false)
     xmodule_cmd = 'xmodule_assets common/static/xmodule'
     if watch
@@ -65,9 +58,6 @@ def coffee_cmd(watch=false, debug=false)
 end
 
 def sass_cmd(watch=false, debug=false)
-    # Make sure to preprocess templatized Sass first
-    preprocess_assets()
-
     sass_load_paths = ["./common/static/sass"]
     sass_watch_paths = ["*/static"]
     if USE_CUSTOM_THEME
@@ -89,6 +79,13 @@ namespace :assets do
     desc "Compile all assets in debug mode"
     multitask :debug
 
+    desc "Preprocess all static assets that have the .mako extension"
+    task :preprocess do
+      # Run assets through the Mako templating engine. Right now we
+      # just hardcode the asset filenames.
+      preprocess_with_mako("lms/static/sass/application.scss.mako")
+    end
+
     desc "Watch all assets for changes and automatically recompile"
     task :watch => 'assets:_watch' do
         puts "Press ENTER to terminate".red
@@ -97,9 +94,9 @@ namespace :assets do
 
     {:xmodule => :install_python_prereqs,
      :coffee => :install_node_prereqs,
-     :sass => :install_ruby_prereqs}.each_pair do |asset_type, prereq_task|
+     :sass => [:install_ruby_prereqs, :preprocess]}.each_pair do |asset_type, prereq_tasks|
         desc "Compile all #{asset_type} assets"
-        task asset_type => prereq_task do
+        task asset_type => prereq_tasks do
             cmd = send(asset_type.to_s + "_cmd", watch=false, debug=false)
             if cmd.kind_of?(Array)
                 cmd.each {|c| sh(c)}
@@ -114,7 +111,7 @@ namespace :assets do
 
         namespace asset_type do
             desc "Compile all #{asset_type} assets in debug mode"
-            task :debug => prereq_task do
+            task :debug => prereq_tasks do
                 cmd = send(asset_type.to_s + "_cmd", watch=false, debug=true)
                 sh(cmd)
             end
@@ -125,7 +122,7 @@ namespace :assets do
                 $stdin.gets
             end
 
-            task :_watch => prereq_task do
+            task :_watch => prereq_tasks do
                 cmd = send(asset_type.to_s + "_cmd", watch=true, debug=true)
                 if cmd.kind_of?(Array)
                     cmd.each {|c| background_process(c)}