Add plumbing to support gradients in OpenGL renderer.

The LinearGradient class keeps a copy of the various parameters that
define the gradient. The copies are native arrays to avoid copying
Java arrays on every draw call. The gradient code path is implemented
until OpenGLRenderer::drawRect() (see TODO.) The actual gradient
implementation will be added in a latter change.

Change-Id: I9300d250ef5e2e9c2e097c3116ee71dfc9d752d8