AP_HAL_Linux: add shrink_8bpp() to VideoIn

This function shrinks a selected area on a 8bpp image.

The focus in this function was the performance, so this may not be the
clearer or the most understandable way to write it. The performance
was measured using GoogleBenchmark[1].

[1] - https://github.com/google/benchmark.git

libraries/AP_HAL_Linux/VideoIn.cpp

@@ -250,6 +250,45 @@ void VideoIn::prepare_capture()
     }
 }
 
+void VideoIn::shrink_8bpp(uint8_t *buffer, uint8_t *new_buffer,
+                          uint32_t width, uint32_t height, uint32_t left,
+                          uint32_t selection_width, uint32_t top,
+                          uint32_t selection_height, uint32_t fx, uint32_t fy)
+{
+    uint32_t i, j, k, kk, px, block_x, block_y, block_position;
+    uint32_t out_width = selection_width / fx;
+    uint32_t out_height = selection_height / fy;
+    uint32_t width_per_fy = width * fy;
+    uint32_t fx_fy = fx * fy;
+    uint32_t width_sum, out_width_sum = 0;
+
+    /* selection offset */
+    block_y = top * width;
+    block_position = left + block_y;
+
+    for (i = 0; i < out_height; i++) {
+        block_x = left;
+        for (j = 0; j < out_width; j++) {
+            px = 0;
+
+            width_sum = 0;
+            for(k = 0; k < fy; k++) {
+                for(kk = 0; kk < fx; kk++) {
+                    px += buffer[block_position + kk + width_sum];
+                }
+                width_sum += width;
+            }
+
+            new_buffer[j + out_width_sum] = px / (fx_fy);
+
+            block_x += fx;
+            block_position = block_x + block_y;
+        }
+        block_y += width_per_fy;
+        out_width_sum += out_width;
+    }
+}
+
 void VideoIn::crop_8bpp(uint8_t *buffer, uint8_t *new_buffer,
                         uint32_t width, uint32_t left, uint32_t crop_width,
                         uint32_t top, uint32_t crop_height)

libraries/AP_HAL_Linux/VideoIn.h

@@ -51,6 +51,11 @@ public:
                   uint32_t width, uint32_t height);
     void prepare_capture();
 
+    static void shrink_8bpp(uint8_t *buffer, uint8_t *new_buffer,
+                            uint32_t width, uint32_t height, uint32_t left,
+                            uint32_t selection_width, uint32_t top,
+                            uint32_t selection_height, uint32_t fx, uint32_t fy);
+
     static void crop_8bpp(uint8_t *buffer, uint8_t *new_buffer,
                           uint32_t width, uint32_t left,
                           uint32_t crop_width, uint32_t top,