docs: update features.txt for GL_ARB_clear_texture with llvmpipe and softpipe
docs: update features.txt for GL_ARB_clear_texture with llvmpipe and softpipe

Signed-off-by: Lars Hamre <9f0270b693f9961c504822735a48473a3adb2f3f@gmail.com>
Reviewed-by: Roland Scheidegger <7d58aee419d6f9201d75517c885374dff5e7d848@vmware.com>
Reviewed-by: Edward O'Callaghan <b035da0d7e151e09fe2005f3a7bc526beb9b3245@folklore1984.net>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler