Let SetViewClear use uint for colorRgba

[WenceyWang]

    /// Set view clear flags.
    ///
    /// @param[in] _id View id.
    /// @param[in] _flags Clear flags. Use `BGFX_CLEAR_NONE` to remove any clear
    ///   operation. See: `BGFX_CLEAR_*`.
    /// @param[in] _rgba Color clear value.
    /// @param[in] _depth Depth clear value.
    /// @param[in] _stencil Stencil clear value.
    ///
    /// @attention C99 equivalent is `bgfx_set_view_clear`.
    ///
    void setViewClear(
          ViewId _id
        , uint16_t _flags
        , uint32_t _rgba = 0x000000ff
        , float _depth = 1.0f
        , uint8_t _stencil = 0
        );

which colorRgba should be unit.