Fix for #7281

The 72x40 OLED requires selecting the internal IREF, as opposed to the default external IREF. This is an undocumented feature in the SSD1306 datasheet, but is present in the SSD1315 datasheet. It's possible this 72x40 OLED is actually using the newer SSD1315 controller. Sending the IREF select command to SSD1306 displays has no effect on them, so I've just added it to the init_display() instead of wrapping in an if width = 72.

I also have a 128x64 OLED using the SSD1315 controller (smaller ribbon cable) and the proposed change has no effect on the display, as the module comes with the correct current limiting resistor. You can use either internal or external IREF and it works the same.

After testing the various display sizes 128x64, 128x32, 72x40, 64x48, 64x32, I noticed all of the smaller displays use a column address offset which is exactly 1/2 of the remaining pixels. eg.

• on a 64px wide display, there's 32px offset each side (32+64+32=128)
• on a 72px wide display, there's 28px offset each side (28+72+28=128)

In show(), rather than hard coding to 32 when the width is 64, I've made it calculate it based on the display width, which was tested on each of the above display dimensions.