shader: fix invalid texture descriptors and add missing format

src/graphics/guest_gpu/gpu_defs.h

@@ -72,6 +72,7 @@ enum class ChannelLayout : uint32_t {
 	k32_32       = 11,
 	k16_16_16_16 = 12,
 	k32_32_32_32 = 14,
+	k5_6_5       = 16,
 	k5_5_5_1     = 17,
 	k4_4_4_4     = 19,
 	kBc1         = 35,