shader float bit widths: restrict to 32 bits #230

In vanilla Vulkan, floating point scalars must be 32-bit.
shaderFloat64 is an optional feature in Vulkan 1.0
shaderFloat16 is an optional feature introduced by extension VK_KHR_shader_float16_int8

For widest availability, the WebGPU shader programming model should be restricted to 32-bit floats.