libgui: Add a test for SRGB surfaces

Add SRGBTest.RenderToSRGBSurface, which validates that passing the
SRGB colorspace attributes to eglCreateWindowSurface actually
results in an SRGB surface by comparing the output values to those
of a standard RGB surface.

Change-Id: I3d5fef8070ed8fa2357ddd1c5fcc849ae3fbd12a