Adds particle systems to sprites

Adds particle systems to sprites
Adds color sequence class
Refactors drawing of textures

R=eseidel@chromium.org

Committed: https://chromium.googlesource.com/external/mojo/+/f9b788edac70bc2991fc24d8dc358ec358ca0b7f

[viktorl, 2015-06-30 00:00:51]

viktorl@google.com changed reviewers:
+ eseidel@chromium.org

[eseidel, 2015-06-30 01:04:50]

Your particles are huge. :)  And your dart style doesn't match my assumptions in
all places, but lgtm.  We will optimize the particle sizes later.  You might
want to consult Hixie's style guide:
https://github.com/domokit/mojo/blob/master/sky/specs/style-guide.md

https://codereview.chromium.org/1216573009/diff/1/sky/sdk/example/game/lib/ga...
File sky/sdk/example/game/lib/game_demo_world.dart (right):

https://codereview.chromium.org/1216573009/diff/1/sky/sdk/example/game/lib/ga...
sky/sdk/example/game/lib/game_demo_world.dart:105: ParticleSystem particles =
new ParticleSystem(_spriteSheet["laser.png"], rotateToMovement: true,
I'm not sure what the dart style is, but I would have indented this like:

particles = new ParticleSystem(
   foo: bar,
   baz: fla,
   yolo: blob
);

[viktorl, 2015-06-30 01:08:33]

Committed patchset #1 (id:1) manually as
f9b788edac70bc2991fc24d8dc358ec358ca0b7f (presubmit successful).