Reduce memory usage in point size clamping tests

Reduce memory usage in point size clamping tests

Win32 builds running dEQP-VK.rasterization.primitive_size.points.*
caused resource errors when using 16384 x 16384 render targes due to
large memory allocations.

This optimization changes the rendering format from R8G8B8A8 to
a single-channel format R8_UNORM, reduces the sizes of the render
targets on the last two test cases to 10240 x 10240 and 9216 x 9216,
and removes one unnecessary result buffer copy to reduce memory
allocation.

Affects:

dEQP-VK.rasterization.primitive_size.points.*

Components: Vulkan

VK-GL-CTS issue: 1649

Change-Id: I5d9f435961f0cd56f8b520ac7e98cece1d4f73d7