Add 8-bit/16-bit transform feedback support for future use

Test/spv.16bitxfb.vert

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+#version 450 core
+
+#extension GL_AMD_gpu_shader_half_float: enable
+#extension GL_AMD_gpu_shader_int16: enable
+
+layout(location = 0) in f16vec4 if16v4;
+layout(location = 1) in i16vec4 ii16v4;
+layout(location = 2) in u16vec4 iu16v4;
+
+layout(location = 0, xfb_buffer = 0, xfb_stride = 6, xfb_offset = 0) out f16vec3 of16v3;
+layout(location = 1, xfb_buffer = 1, xfb_stride = 6, xfb_offset = 0) out F16Out
+{
+    float16_t of16;
+    f16vec2   of16v2;
+};
+
+layout(location = 5, xfb_buffer = 2, xfb_stride = 6, xfb_offset = 0) out i16vec3 oi16v3;
+layout(location = 6, xfb_buffer = 3, xfb_stride = 6, xfb_offset = 0) out I16Out
+{
+    uint16_t ou16;
+    u16vec2  ou16v2;
+};
+
+void main()
+{
+    of16v3 = if16v4.xyz;
+    of16   = if16v4.x;
+    of16v2 = if16v4.xy;
+
+    oi16v3 = ii16v4.xyz;
+    ou16   = iu16v4.x;
+    ou16v2 = iu16v4.xy;
+}