Allow alternative mix() implementation
Allow alternative mix() implementation

Some compilers translate mix(a, b, x) to a + (b - a)*x which produces
slightly different results than the form a*(1 - x) + b*x suggested by
the GLSL specification. This change relaxes tests to allow both forms.

Bug: 21110253
Change-Id: I3af056145ef8e64c0fc998302770fd38d105481a

chadversary/deqp,KhronosGroup/VK-GL-CTS,KhronosGroup/VK-GL-CTS,chadversary/deqp,googlestadia/VK-GL-CTS,googlestadia/VK-GL-CTS,KhronosGroup/VK-GL-CTS,KhronosGroup/Vulkan-CTS,KhronosGroup/VK-GL-CTS,chadversary/deqp,KhronosGroup/Vulkan-CTS,KhronosGroup/VK-GL-CTS,KhronosGroup/Vulkan-CTS,chadversary/deqp,googlestadia/VK-GL-CTS,geekboxzone/mmallow_external_deqp,KhronosGroup/Vulkan-CTS,geekboxzone/mmallow_external_deqp,KhronosGroup/VK-GL-CTS,geekboxzone/mmallow_external_deqp,googlestadia/VK-GL-CTS,geekboxzone/mmallow_external_deqp,chadversary/deqp,chadversary/deqp,KhronosGroup/Vulkan-CTS,googlestadia/VK-GL-CTS,geekboxzone/mmallow_external_deqp,geekboxzone/mmallow_external_deqp,KhronosGroup/Vulkan-CTS,googlestadia/VK-GL-CTS