This adds support for GL_ARB_texture_rectangle, as it is used in Chromium.
This is required in order to use EGL/GLES on MacOS using IOSurface,
in order to be able to run Chromium on top of SwiftShader on MacOS.

Change-Id: I3c0b6a137892583bbfbc68149874d5bec3026b4a
Reviewed-on: https://swiftshader-review.googlesource.com/16368Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>