Implementing missing x11_rawfb features colored text and multi-color-rect

[0x0203]

This implements rendering of colored text for the x11_rawfb demo and an implementation of rendering multi-colored rectangles. The latter is used by the color picker widget which currently doesn't display properly.

Both of these changes make use of the nk_image_blendpixel() routine which I modified to swap the r and b bytes of the nk_color. I'm still not sure where these are getting swizzled in the first place, but this change makes it so everything renders in the requested color. If anyone has any insight into where the 'real' fix for this is, I can make the appropriate change.

[0x0203]

Okay, I think I've got this properly fixed now. As part of fixing this, I made some other changes that made things more consistent, clear, and correct.

• The main fix was to make sure getpixel() and setpixel() read and write the pixel data in the same way.
• There were fencepost errors in the get/set pixel routines that prevented them from working on row/column 0.
• Setting text color now applies all three color channels on any part of the font atlas image that is not 100% transparent.
• I made the names of the routines consistent to differentiate the functions that operate on a struct rawfb_image from those that deal with a struct rawfb_context.
• Fixes in main.c for the commented out set_style() function were made so that they would work with the rawfb context once uncommented.
• Finally, to reduce confusion, I made the #define for the pixel ordering in main.c match that defined in the Makefile.

[0x0203]

Thanks for the feedback; should now all be addressed.

[dumblob]

Closing as per authors request - see https://github.com/vurtun/nuklear/pull/847#discussion_r290946425 . There shall be another extended pull soon from the author.