GL allows reads beyond the defined pixel rectangle, although their
resultant values are undefined. D3D11 gives an error when reading
out-of-bounds, hence clamping the area satisfies both APIs.

This fixes a Renderer crash in our ReadPixels out of bounds test,
and also fixes the test to test the correct area of pixels.

BUG=angle:590

Change-Id: I12439c22d53ec6a2d69e1b8cf80f53e9c18e11f7
Reviewed-on: https://chromium-review.googlesource.com/191080Reviewed-by: Shannon Woods <shannonwoods@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>