Microsoft documentation says:
> If biCompression equals BI_RGB and the bitmap uses 8 bpp or less, the bitmap has a color table immediatelly following the BITMAPINFOHEADER structure. The color table consists of an array of RGBQUAD values. The size of the array is given by the biClrUsed member. If biClrUsed is zero, the array contains the maximum number of colors for the given bitdepth; that is, 2^biBitCount colors.
Formerly, we treated 0 colors as "no image palette" during construction,
but then during common_hal_displayio_ondiskbitmap_get_pixel indexed into
the palette anyway. This could have unpredictable results. On a pygamer,
it gave an image that was blue and black. On magtag, it gave a crash.
The transparent_color field was never initialized. I _think_ this means
its value was always set to 0, or the blackest of blacks. Instead,
initialize it to the sentinel value, newly given the name
NO_TRANSPARENT_COLOR.
This exposed a second problem: The test for whether there was an existing
transparent color was wrong (backwards). I am guessing that this was not
found due to the first bug; since the converter had a transparent color,
the correct test would have led to always getting the error "Only one
color can be transparent at a time".
Closes#3723
* Initialize the EPaper display on the MagTag at start.
* Tweak the display send to take a const buffer.
* Correct Luma math
* Multiply the blue component, not add.
* Add all of the components together before dividing. This
reduces the impact of truncated division.
@cwalther determined that for boards with 2 displays (monster m4sk),
start_terminal would be called for each one, leaking supervisor heap
entries.
Determine, by comparing addresses, whether the display being acted on
is the first display (number zero) and do (or do not) call start_terminal.
stop_terminal can safely be called multiple times, so there's no need
to guard against calling it more than once.
Slight behavioral change: The terminal size would follow the displays[0]
size, not the displays[1] size
A call to supervisor_start_terminal remained in
common_hal_displayio_display_construct and was copied to other display
_construct functions, even though it was also being done in
displayio_display_core_construct when that was factored out.
Originally, this was harmless, except it created an extra allocation.
When investigating #3482, I found that this bug became harmful,
especially for displays that were created in Python code, because it
caused a supervisor allocation to leak.
I believe that it is safe to merge #3482 after this PR is merged.
An RGBMatrix has no bus and no bus_free method. It is always possible
to refresh the display.
This was not a problem before, but the fix I suggested (#3449) added
a call to core_bus_free when a FramebufferDisplay was being refreshed.
This was not caught during testing.
This is a band-aid fix and it brings to light a second problem in which
a SharpDisplay + FrameBuffer will not have a 'bus' object, and yet does
operate using a shared SPI bus. This kind of display will need a
"bus-free" like function to be added, or it can have problems like
#3309.
