Implement frame buffer

[fornellas]

Implements an optional frame buffer on top of existing functionality. It works by redirecting all drawing functions to happen in memory, and allow users to explicitly send the buffer as a whole to the display when finished. This eliminates all visual artifacts resulting from updating graphics directly on the display memory.

Given that not all use cases have enough memory for a frame buffer, users can optionally use it with:

  ucg_InitBuffer(
    &ucg,
    ucg_dev_ssd1351_18x128x128_ilsoft,
    ucg_ext_ssd1351_18,
    ucg_com_cm3_4wire_HW_SPI
  );
  ucg_SetColor(&ucg, 0, 255, 255, 255);
  ucg_DrawBox(&ucg, 0, 0, 100, 10);
  ucg_SendBuffer(&ucg);

Data will be all pumped to the display only when ucg_SendBuffer is called.

I tested this with a SSD1351 and the SDL examples, everything seems all right.

Implementation details:

• ucg_InitBuffer puts a new ucg_dev_buffer in place of ucg->device_cb.
• ucg_dev_buffer catches all drawing calls and draws them them in memory.
• ucg_SendBuffer temporarily puts the original device callback and invokes it with the new UCG_MSG_SEND_BUFFER command.
• UCG_MSG_SEND_BUFFER has a default implementation at ucg_dev_default_cb.c which all displays can use.
• Because this is not fast, each display driver can / should have its own optimized implementation.
• I added UCG_MSG_SEND_BUFFER to ucg_dev_ic_ssd1351.c, which is able to send the buffer at bus speed.
• Similar few lines implementation for other displays should be easy, however I don't have them at hand to test any changes.

Closes #122.

PS: This PR is on top of #123, which should be reviewed / merged first.

[fornellas]

I'm closing this PR as it seems there's no interest in merging or reviewing it. I ended up writing a completely new graphics library, which solves this an many other problems: https://github.com/fornellas/eglib.