Support manually packing constant buffer elements through the `packoffset(·)` syntax.
Support not included yet for simultaneously having semantics, `register(·)`, and `packoffset(·)`, for abnormalities such as: ```hlsl Texture2D tex;
cbuffer buff { float4 a : packoffset(c0); sampler sam : packoffset(c0) : register(s1) : SEMANTIC; }
float4 main() : sv_target { return tex.Sample(sam, float2(0, 0)) + a; } ``` but this motivated the addition of the `hlsl_ir_var.offset_reservation` field instead of reusing `hlsl_ir_var.reg_reservation`.
-- v6: vkd3d-shader/hlsl: Consider register() as manual packing for resource fields. tests: Test packoffset() with resources inside cbuffers. vkd3d-shader/hlsl: Ignore packoffset() contents for SM1. vkd3d-shader/hlsl: Don't allow manual and automatic cbuffer offset packing. vkd3d-shader/hlsl: Detect overlaps in cbuffer offsets. vkd3d-shader/hlsl: Support packoffset(). vkd3d-shader/hlsl: Parse packoffset(). vkd3d-shader/hlsl: Rename struct hlsl_reg_reservation fields. tests: Test packoffset(). tests: Test cbuffer element offsets.