Loosen color target tolerances for some tests

Loosen color target tolerances for some tests

Some tests verify the results of glReadPixels within two bits of
tolerance. However, some pixel formats (e.g., RGB 565) cannot support
such precision. Loosen hard-coded tolerances, such that tests can pass
with 5-bit color channels.

Additionally change the types of some variables from GLubyte to int. Log
messages constructed through stream operators would treat such values as
characters, rather than numeric quantities.

Components: OpenGL

VK-GL-CTS issue: 950

Affects:
KHR-GL46.indirect_parameters_tests.MultiDrawArraysIndirectCount
KHR-GL46.indirect_parameters_tests.MultiDrawElementsIndirectCount
KHR-GL46.shader_atomic_counter_ops_tests.*
KHR-GL46.shader_viewport_layer_array.*

Change-Id: I7da30842004caf1698d83128678c1bda82fff787