setRect x and y is unsigned, but opengl, metal support negative

[CatDroid]

in bgfx_p.h, it limits ViewPort 's x and y only 0 or positive , but in opengl and metal , it is negative allowed .

why do so limit ?

`
BX_ALIGN_DECL_CACHE_LINE(struct) View

void setRect(uint16_t _x, uint16_t _y, uint16_t _width, uint16_t _height)
{
m_rect.m_x = uint16_t(bx::max<int16_t>(int16_t(_x), 0) );
m_rect.m_y = uint16_t(bx::max<int16_t>(int16_t(_y), 0) );
m_rect.m_width = bx::max<uint16_t>(_width, 1);
m_rect.m_height = bx::max<uint16_t>(_height, 1);
}

`