Add method to create Font from (const) string

[mcder017]

...rather than requiring fonts to be loaded through a File handle. New ReadFont method is useful for allowing default built-in font simply using a header file with a defined string constant (e.g. BDF_10X20)

[mcder017]

Users can then create a header file, such as bdf_10x20_local.h, that has content:

#ifndef BDF_10X20_LOCAL_H
#define BDF_10X20_LOCAL_H
const char* BDF_10X20_STRING = R"BDF_NON_KEYWORD(
---copy-paste the entire bdf font file here, from the STARTFONT line all the way through to the ENDFONT line---
)BDF_NON_KEYWORD";
#endif //BDF_10X20_LOCAL_H

and in the program itself, #include the header file and then load the font programmatically (no file access):

  rgb_matrix::Font smallFont;
  if (!myFont.ReadFont(BDF_10X20_STRING)) {
    fprintf(stderr, "Couldn't read internal font '%s'\n", BDF_10X20_STRING);  // should never happen if bdf font valid when pasted
  }

[mcder017]

@hzeller, merged in the recently approved 1-line change (to respect bbx offset) that WardBrian had resolved.