GL_SRG8_ALPHA8 also supported in OpenGL ES 3.0+

GL_SRG8_ALPHA8 also supported in OpenGL ES 3.0+

dEQP-GLES2.functional.fbo.completeness.renderable.renderbuffer.color0.srgb8_alpha8
assumes that using GL_SRGB8_ALPHA8 format in RenderBuffer should fail
for OpenGL ES 2.0 because it is not supported in the specification,
unless either EXT_sRGB or EXT_sRGB_write_control are implemented, as
they add this format as supported.

But when creating the OpenGL context, it can be either 2.0 or an upper
version. If 3.0+ is returned, this version supports this format without
those extensions, and thus the test should pass.

So this change ensures that if OpenGL ES 3.0+ context is returned, then
the use of GL_SRGB8_ALPHA8 in RenderBuffer should work.

Affected tests:
dEQP-GLES2.functional.fbo.completeness.renderable.renderbuffer.color0.srgb8_alpha8

Components: OpenGL

VK-GL-CTS issue: 1137

Change-Id: Ibb95736959ff9d5b0ff972ccf3983abc98795be3