Move sdl and gl into the ui directory.

These both only deal with the user interface, so let's put them there.
Use float32 for SetUniform.
Add the Canvas type.

It provides a high-level API for 2d graphics.
Use reflection to determine the value types in SetUniform.
Add support for drawing images.
Add ui.LoadPng.
Reorder some functions.
Use better names for Canvas fields.
Add ui.StrokeLine.