Lanai: Optimize index calculation while copying glyphs

At many places where we are using replaceRegion to copy glyph into MTLTexture we have used literals for index update.

Update them to use loop increment like :
for (int i = 0; i < (w * h); i++) {
                    imageData[dstIndex++] = 0xFF;
                    imageData[dstIndex++] = glyph->image[srcIndex++];
                    imageData[dstIndex++] = glyph->image[srcIndex++];
                    imageData[dstIndex++] = glyph->image[srcIndex++];
}