Fix three categories of test bugs in the kitty graphics C API tests:
The placement iterator reset in getTyped was clobbering the
layer_filter field when reinitializing the iterator struct,
causing the layer filter test to see unfiltered placements.
Preserve layer_filter across resets.
The viewport position tests were not accounting for the default
cursor_movement=after behavior of the kitty display command,
which calls index() for each row of the placement before the
test scroll sequence. Add C=1 to suppress cursor movement so
the scroll math in the tests is correct.
The source_rect tests used an 88-character all-A base64 payload
which decodes to 66 bytes, but a 4x4 RGBA image requires exactly
64 bytes. Fix the payload to use proper base64 padding (AA==).
65e3265e3c