Bind renderbuffers as soon as they are generated.
Bind renderbuffers as soon as they are generated.

Like textures, renderbuffers don't become real objects until they are
bound.  If the renderbuffer is never bound before being attached to a
framebuffer, it would generate an INVALID_OPERATION.

Fixes:
 * conformance/textures/tex-input-validation.html
 * conformance/renderbuffers/renderbuffer-initialization.html

BUG=angleproject:886

Change-Id: I639b9e2d9ae694dce29f20ed5f95077ae02d471e
Reviewed-on: https://chromium-review.googlesource.com/273533
Reviewed-by: Corentin Wallez <470c533e86f4f848c98c7e11fffc2d043494a401@chromium.org>
Reviewed-by: Jamie Madill <7e492b4f1c8458024932de3ba475cbf015424c30@chromium.org>
Tested-by: Geoff Lang <b6fc25fe0362055230985c05cbfa8adb741ccc0f@chromium.org>

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