d3d11: Shader code is not compatible with `ps_4_0_level_9_3` profile

Current HLSL code in d3d11 plugin is not compatible with D3D_FEATURE_LEVEL_9_3 or lower.

Some VMs don't support higher d3d feature level so current shader code would not be compiled.

0:00:00.679937000 50108 0000027F32E82A40 ERROR            d3d11shader gstd3d11shader.c:67:compile_shader: could not compile source, hr: 0x80004005, error detail C:\Work\gst-build\Shader@0x0000027F32C73120(21,20-71): warning X3206: 'Sample': implicit truncation of vector type
C:\Work\gst-build\Shader@0x0000027F32C73120(18,14-30): error X3508: 'main': Top-level return value is not completely initialized
C:\Work\gst-build\Shader@0x0000027F32C73120(18,14-30): error X3508: 'main': Top-level return value is not completely initialized
C:\Work\gst-build\Shader@0x0000027F32C73120(18,14-30): error X3508: 'main': Top-level return value is not completely initialized
C:\Work\gst-build\Shader@0x0000027F32C73120(18,14-30): error X3508: 'main': Top-level return value is not completely initialized

The code should be re-written somehow or we need to revert !1311 (merged) if we cannot fix it before 1.18 release.