VideoCommon: add additional data types to material asset

[iwubcode]

This is another PR for adding support for custom shader uniforms to Dolphin. This time, we extend the Material asset to include additional properties (int, int2, ... float, float2,...bool) and add some helper functions to get the material data to the GPU (not used yet, will be used in a future PR).

[TellowKrinkle]

Strongly typed variants are cool, but I also think you could get a lot less code duplication if you made the element count a runtime value
e.g. WriteAsShaderCode could be closer to

static const char* to_name(MaterialProperty::Type type)
{
  switch (type)
  {
    case MaterialProperty::Type::Type_Int:   return "int";
    case MaterialProperty::Type::Type_Float: return "float";
    case MaterialProperty::Type::Type_Bool:  return "bool";
  }
}
void MaterialProperty::WriteAsShaderCode(ShaderCode& shader_source, const MaterialProperty& property)
{
  const char* type_name = to_name(property.type);
  std::string elems = property.element_count == 1 ? "" : std::to_string(property.element_count);
  shader_source.Write("{}{} {};\n", type_name, elems, property.m_code_name);
  for (int i = property.element_count; i < 4; i++)
    shader_source.Write("{} {}_padding_{};\n", type_name, property.m_code_name, i + 1);
}

I think it would also make a future change where we start tightly packing elements (instead of padding everything to 16 bytes) to save space much easier.

[iwubcode]

The material now uses only std::variant as opposed to also including a type. I updated the material data to also not use std::optional as data should always be present (eventually will be defaulted as laid out by shader data).