[feat] bump base for SDL2: update parts of window like a real device (#3812)

https://github.com/koreader/koreader-base/pull/626

This commit uses a viewport to draw only a subset of the blitbuffer onto the
rectangle to be updated in the SDL texture.

Without this, there are cases where a full screen (or at least bigger) refresh
is required that will not be obvious without first testing on a real device.
See https://github.com/koreader/koreader/pull/3804

Also fixes `EMULATE_READER_FLASH`, which was accidentally broken in 981bd40e1c

Also includes: [fix] SDL2: apply night mode on window resize (https://github.com/koreader/koreader-base/pull/627)

base

@@ -1 +1 @@
-Subproject commit 7cb11c4f52787207264d2284213240b4c065e27b
+Subproject commit 2ecc6d1134ae94053653924648d7a04945428616