Minor improvements

[aremmell]

• Delete default constructor, so that people don't accidentally default-construct an instance which will have garbage values for all members (including CS and RST pins)
• Implement virtual destructor (derived class). Actually, now that I think about it, Adafruit_GFX needs a virtual destructor as well. I'll submit a different PR for that
• Provide default initialization values for members which will remain uninitialized with garbage values until begin() is called (I found out the hard way when I tried to get width and height before calling begin())
• Remove unnecessary if-statement (an unsigned 8-bit value cannot be greater than 255, so the capping is superfluous)