These functions enable mixing base and grid graphics in
aheatmap, by avoiding calls to the grid
'L_gridDirty'. They are not
exported (i.e. not tampering core functions) and are only
meant for internal use within the NMF package.
tryViewport tries to go down to a viewport in the
current tree, given its name.
current.vpPath_patched aims at substituting
current.vpPath, so that the graphic
engine is not reset. This is essentially to prevent
outputting a blank page at the beginning of PDF graphic
.use.grid.patch tells if the user enabled patching
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seekViewport as the latter would reset
the graphic device and break the mix grid/base graphic