add channels test (migrate from drishti)

[headupinclouds]

add separate api and private/unit test executables Related: #94

We need a separate executables for: (1) the public (possibly shared) ACF api (2) the private support class/functions used internally in ACF that require focused testing

• add SIMD channel conversion code to ACF (imgrated from drishti) and to private unit test
• move optimized separable triangle filter shader test to private unit test
• extract ogles_gpgpu::ACF::getImage() texture read code to acf/transfer.{h,cpp} as stand-alone functions for easier reuse (without require compilatino of GPUACF.{h,cpp}
• collect private unit test sources with sugar in ACF_TEST_SRCS variable and build into the acf unit test executable