To create a 2x2 raster block with one byte per pixel:
```
block = QgsRasterBlock(Qgis.Byte, 2, 2)
block.setData(b'\xaa\xbb\xcc\xdd')
```
To overwrite existing raster data at position 0,0 by the 2x2 block:
```
provider.setEditable(True)
provider.writeBlock(block, band, 0, 0)
provider.setEditable(False)
```
And overriding projection is only done if user has manually
chosen a different CRS from the list.
This means that templates and compositions in server can still
adapt to follow the project CRS, but if desired specific map
items (eg an overview map) can use the CRS override to always
show in a particular CRS.
This allows the CRS for map items to differ from the canvas/project
CRS. It also allows different map items to have different CRS,
eg an overview map can be set to a different CRS to the main map.
An unfortunate side effect of this change and the ongoing work
to separate compositions from canvas is that datum transforms
are no longer supported in composer. This cannot be fixed until
the datum transform store is rewritten to not depend on canvas
(ie, it's also broken for upcoming multi-canvas work)
Use relevant composer map item CRS instead of composition mapsetting's
CRS. For items directly linked to a map (eg scalebar, legend) we
use that map's CRS, for others (eg labels, html items) we use the
composition's reference map CRS.
(Note that the CRS cannot be set for a map, and is still currently
linked to the canvas CRS. Fixing that is a future TODO.)
This allows pieces of code that depend on map layers to listen directly
to the layer's notification rather than having to listen to project's
layersWillBeRemoved signal (and cycle through the whole list)
These were not being used by QGIS code (always left at default 1.0
value), and removing them from the api allows us to simplify lots
of code. It also makes QgsRenderContext scaling much less confusing.
This method was used to sync the canvas layers to a composer map.
The logic has been moved to app to avoid a dependancy between
compositions and the main map canvas
draw shape/arrow/nodes/paper symbols
This commit removes some more instances where changes in the
canvas affect compositions. Previously the symbols drawn
in shape/arrow/etc items were using the scale from the canvas.
This meant that the appearance of these items in composer
would change depending on canvas zoom if they used symbols
with map unit sizes.
Now they take their map scale from the composition's reference
map. While this has the nice side effect that now map units
can be used in the appearance of these items and they're
guaranteed to match up with the reference map item, the main
intention here is to remove more of the forced links between
compositions (core) and the main canvas (app).
This option will be used for more than just world file generation,
and instead will indicate which map should be considered the master
map for the composition from which any composer properties
calculating map units will derive the scale from.
