Disable warning of rasterization after 100k points in DimPlot, FeaturePlot etc.

[Peter-bbh]

When number of points to plot exceeds 100,000, the plot is defaulted to rasterization and a warning is issued, "Rasterizing points since number of points exceeds 100,000. To disable this behavior set raster=FALSE".

First time it is great, but always is anoying. It can be removed by suppressWarnings(), see #6802, but it clutters the code.

An easy fix seems to be to change the preceeding condition in "visualization.R" (three places) from
"if ((nrow(x = data) > 1e5) & !isFALSE(raster)){" to "if ((nrow(x = data) > 1e5) & is.null(raster)){"

In this way, only if the raster option is not already specified, the warning will appear. So setting "raster = TRUE" will produce the same plot, but without the warning.

[samuel-marsh]

@Peter-bbh this is now to be implemented in Seurat v4.4 (release soon). See #7842.

Best,
Sam