Refactor the oled_i2c example (#217)

This PR does a number of things:

1. Renames the oled_i2c example to sssd1306_i2c to match other example folder
   names that use the device name
2. Merge code from another ssd1306 exmaple that was sitting in another PR
3. Add a few more demo parts to the code for extra fun.
4. Couple of bug fixes that were reported in original code to fix handling different
   display sizes, so code now works for 32 and 64 pixel height displays by changing
   a define at top of code.

i2c/ssd1306_i2c/CMakeLists.txt

@@ -0,0 +1,12 @@
+add_executable(ssd1306_i2c
+        ssd1306_i2c.c
+        )
+
+# pull in common dependencies and additional i2c hardware support
+target_link_libraries(ssd1306_i2c pico_stdlib hardware_i2c)
+
+# create map/bin/hex file etc.
+pico_add_extra_outputs(ssd1306_i2c)
+
+# add url via pico_set_program_url
+example_auto_set_url(ssd1306_i2c)