Ran a test on a feather_nerf52840 with a TFT featherwing --If I configure displayio then run the stmpe610 test using SPI, the stmpe610 test runs normally, but it causes the display to be deactivated -- that is I no longer see REPL output on the screen and even after a soft reboot, I can't get the REPL to show up on the screen again. If I run the slideshow - it does not show any images. No erros, but no images. If I hard reset, the slideshow (non-sd) runs normally. The slideshow from the SD card runs as above, no errors, but no images.

Press any key to enter the REPL. Use CTRL-D to reload.
Adafruit CircuitPython 4.0.0-beta.2-200-gc854f6617 on 2019-03-06; Adafruit Feather nRF52840 Express with nRF52840
>>> import slideshow_sd
cats1.bmp            Size: 37.0 KB
IMG_0493.bmp         Size: 37.0 KB
IMG_1027.bmp         Size: 66.2 KB
skye1.bmp            Size: 46.2 KB
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "slideshow_sd.py", line 56, in <module>
  File "adafruit_slideshow.py", line 200, in __init__
  File "adafruit_slideshow.py", line 318, in advance
KeyboardInterrupt: 

on a pyportal slideshowsd seems to run fine

also as in #1612, with this PR I can still mount/access the SDCard with the displayio device active -- teh REPL is echoing all the activity running a script from the SDCard.

as suggested by @dhalbert Tried adding a delay in the stmpe610 read_data loop. Added a .25 sec delay -- this works -- the display continues to echo the REPL throughout the test and after the program is stopped with a control-c.

@jerryneedell I fixed some remaining problems, and tested on PyPortal and a TFT FeatherWing with Feather M4 reading from SD card. Seems to work now. Might work better with STMPE also.

Feel free not to test tonight :)

@tannewt Besides some general cleanup, this fix locks the SPI bus as necessary, but unlocks it between buffer writes, which gives the SD card code the access to read image data.

Tested on feather_nrf52840 with TFT featherwing:
slideshow from SD Card worked normally!
stmpe610 test worked with one issue that is more a problem with the test than CP.
When the displayio device is in use, there is a noticable slowdown in the execution speed at the REPL. You can see it just typing. the response is sluggish.
For a device like the stmpe610 that buffers data quickly, this resulted in some significant lag issues. But that will likely have to be addressed by the user code.

The "sluggishness" is not new -- It has always been present when using displayio on the feather_nrf52840.

Do you see the same on the feather_m4? - I have not tried it recently.

tested on pyportal -- slideshow from SDCard worked normally
aslo tested mounting and executing a script from the SDCard -- worked fine
Note: the responsiveness of the REPL in the PyPortal is much better than on the feather_nrf52840.
The "sluggishness" mentioned above is not apparent.

This PR looks good to me -- I see it failed the German Metro M0 build on Travis as I found yesterday :-(

Travis is confused: both builds have finished successfully, but it hasn't noticed.