Fix #15192: Row grouping breaks after first page

[marcoschndlr]

Fixes #15192

What was the problem

shouldRenderRowGroupHeader expects the rowIndex to be the absolute index (i.e. first + rowIndex) when paginated.
The problem was that the rowIndex passed starts from 0 for every page, and the index calculation in the method then resulted in a negative number.

How was the problem solved

For expanded rows, the method getRowIndex is used, which adds the starting offset (first) to the rowIndex.
This method is used here now as well.

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Name	Status	Preview	Comments	Updated (UTC)
primeng	⬜️ Ignored (Inspect)	Visit Preview		Aug 29, 2024 7:30am

[nckirik]

this should be also applied to group footers

also shouldRenderRowGroupFooter method is calculating the wrong index.

it should be like this

    shouldRenderRowGroupFooter(value: any, rowData: any, i: number) {
        let currentRowFieldData = ObjectUtils.resolveFieldData(rowData, this.dt.groupRowsBy);
//        let nextRowData = value[i + (1 + this.dt._first)];
        let nextRowData = value[i + (1 - this.dt._first)];
        if (nextRowData) {
            let nextRowFieldData = ObjectUtils.resolveFieldData(nextRowData, this.dt.groupRowsBy);
            return currentRowFieldData !== nextRowFieldData;
        } else {
            return true;
        }
    }