opengl-cts-4.6.8.0: OpenGL CTS 4.6.8.0

New Tests:
 * Update the SHA of gl.xml for GL_EXT_fragment_shading_rate desktop OpenGL support
 * Add KHR-GL*.fragment_shading_rate tests

Fixes:
 * GLES: Avoid precision-related errors in rgb_to_hsl_vertex
 * Fix clearTexImage tests
 * Fix EXT_mesh_shader tests buffer access after unmap
 * Fix typo caused GS/VS definition on Group 25 test 1
 * Correct output for direct_state_access.buffers_clear when failure occurs
 * Correct glGetUniformSubroutineuiv return value comparison logic
 * Check ssbo limits before ssbo array test
 * Skip formats that are not supported in GL Core context
 * Fix SIGNED_INT_1010102_REV handling in tcuTexture
 * Check spirv support before spirv capabilities test
 * Remove Resource::getBuffer
 * Fixed the error in the return value type of deProcess_create
 * Fix SyntaxWarning due to invalid escape sequence in gen_framework.py
 * Upgrade libpng
 * Document that -n allows multiple patterns by using commas
 * Fix RenderDoc library name on macOS to support captures
 * Add -v option to fetch_sources for compatibility in CI
 * Waiver file format documentation and validation