grx_context_bit_blt_1bpp only supports left-most pixel in the most-significant byte

[dlech]

grx_context_bit_blt_1bpp() only supports the convention of .bmp files where the left-most pixel in the most-significant bit of each byte.

However, file formats such as .xbm have the left-most pixel as the least-significant byte. As a result these are drawn with with each 8 pixel column reversed.

We can either add a parameter to grx_context_bit_blt_1bpp() or create a second function to handle reversal of the bits.

This is probably the most efficient way to reverse the bits since we are primarily targeting 32-bit systems.