Add tests for EXT_discard_rectangles

Add tests for EXT_discard_rectangles

Added tests that check that the rendered image is correct for the the
combination of: the two different modes (inclusive, exclusive),
different number of rectangles (1, 2, 3, 4, 8, 16), dynamic discard
rectangles, scissor, dynamic scissor.

These tests don't check that the device groups work as expected.

New tests:

dEQP-VK.draw.discard_rectangles.*

Components: Vulkan
VK-GL-CTS issue: 1679

Change-Id: Idcfe2a9fc9113051fd72e51a0019e4247463cede