Add fillTextRect() function for alignment, clip and bg fill.

[PetteriAimonen]

Added new method fillTextRect().

It handles the following:

• Aligns text horizontally and vertically: centered / left / right / top / bottom.
• Clips any part of text that would go outside rectangle.
• Fills background with minimal flicker, with correct handling for overlapping parts of custom fonts.

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Risks / limitations / drawbacks:

• No modification to existing functions.
• Does not currently handle text scaling or wrapping. These would be possible to add in the future as an overload if someone needs them.
• Quite large function in code size (about 1kB on ARM), but if it is not called it gets optimized away.

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Example test:

display.setFont(&FreeSans18pt7b);
display.fillTextRect("Top left\nLong line is longer than long cat\nShort", 10, 10, 190, 140, Adafruit_GFX::TOP_LEFT, 0xFFFF, 0x3333);
display.fillTextRect("Mid center\nLong line is longer than long cat\nShort", 10, 160, 190, 140, Adafruit_GFX::MID_CENTER, 0xFFFF, 0x3333);       
display.fillTextRect("Bottom right\nLong line is longer than long cat\nShort", 210, 160, 190, 140, Adafruit_GFX::BOT_RIGHT, 0xFFFF, 0x3333);
display.setFont(&Z003_MediumItalic20pt7b);
display.fillTextRect("Overlap\nworks also", 210, 10, 190, 140, Adafruit_GFX::MID_CENTER, 0xFFFF, 0x3333);

gives the following output:

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Regarding issue #223: This function handles the problem of overlapping glyph parts by tracking the largest x and y of glyphs rendered so far. The background is written only in new area. Flicker only occurs in the overlapping parts of glyphs, otherwise each pixel is only written once.

[PetteriAimonen]

If there are any concerns or questions about this, feel free to ask me. I think this could be quite useful to get into the code.

[PetteriAimonen]

@makermelissa Just a ping, would you be interested in taking a look at this pull request? If I understand correctly, you work at Adafruit and had previously commented on issue #223.