fix: SkiaOutputDeviceOffscreen use GL_TEXTURE_RECTANGLE_ARB when
SkiaOutputSurface use it


Currently SkiaOutputSurface will create a characteristic that backend_format
will be kRectangle if the build is for macOS.
But when the output_device is SkiaOutputDeviceOffscreen, the surface check
will fail at GrSurfaceProxy::validateSurface saying:
surface->asTexture()->textureType() != this->textureType()

This fix make SkiaOutputDeviceOffscreen use same backend_format when
creating Backbuffers.

Change-Id: I959c96cb61d85c2358c47dc70d90ea1396f60d93
Bug: 324934085
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5266558
Reviewed-by: Kyle Charbonneau <kylechar@chromium.org>
Reviewed-by: Vasiliy Telezhnikov <vasilyt@chromium.org>
Commit-Queue: Kyle Charbonneau <kylechar@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1260091}