Rename display_{size,width,height} to render_*.

The name "display_*" (or "d_*") is used for non-compatible information
(that is, the cropped frame dimensions in pixels, as opposed to the
intended screen rendering surface size). Therefore, continuing to use
display_* would be confusing to end users. Instead, rename the field
to render_*, so that struct vpx_image can include it.

Change-Id: Iab8d2eae96492b71c4ea60c4bce8121cb2a1fe2d