SampleApp: Cleanup SkOSWindow_SDL

Implements:
 * Event loop event waiting (no busy loop)
 ** Skia timers
 * Proper window sizing and resizing
 * MSAA on X11 (previously it did not use MSAA at all)
 * Changes requested colorspace from 565 to 8888
   to match non-SDL variants
 * Context creation clearing (color and stencil mask)
 * Opens the window in 640x480 by default on desktop.

Removes dead code.

BUG=skia:4733
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1578173002

Review URL: https://codereview.chromium.org/1578173002
2 files changed