ipu3: cio2: Tweak sensor size selection policy

Do not compare higher precision of the ratios, as it might lead to
absurd selection of sensor size for a relatively low requested
resolution size.

For example:
The imx258 driver supports the following sensor resolutions:

 - 4208x3118 = 1.349583066
 - 2104x1560 = 1.348717949
 - 1048x780  = 1.343589744

It can be inferred that, that the aspect ratio only differs by a small
mantissa with each other. It does not makes sense to select a 4208x3118
for a requested size of say 640x480 or 1280x720, which is what is
happening currently.
($) cam -c1 -swidth=640,height=480,role=raw
    - CIO2 configuration: 4208x3118-SGRBG10_IPU3 [*]

In order to address this constraint, only compare the ratio with single
precision to make a better decision on the sensor resolution policy
selection.

($) cam -c1 -srole=raw,width=640,height=480
    - CIO2 configuration: 1048x780-SGRBG10_IPU3 [*]

[*] Please revert 0536a9aa7189("ipu3: Disallow raw only camera configuration")
    if the configuration is reported as invalid.

Signed-off-by: Umang Jain <umang.jain@ideasonboard.com>
Tested-by: Jacopo Mondi <jacopo@jmondi.org>
Reviewed-by: Jacopo Mondi <jacopo@jmondi.org>
Reviewed-by: Kieran Bingham <kieran.bingham@ideasonboard.com>

src/libcamera/pipeline/ipu3/cio2.cpp

@@ -295,6 +295,14 @@ V4L2SubdeviceFormat CIO2Device::getSensorFormat(const std::vector<unsigned int>
 				continue;
 
 			float ratio = static_cast<float>(sz.width) / sz.height;
+			/*
+			 * Ratios can differ by small mantissa difference which
+			 * can affect the selection of the sensor output size
+			 * wildly. We are interested in selection of the closest
+			 * size with respect to the desired output size, hence
+			 * comparing it with a single precision digit is enough.
+			 */
+			ratio = static_cast<unsigned int>(ratio * 10) / 10.0;
 			float ratioDiff = fabsf(ratio - desiredRatio);
 			unsigned int area = sz.width * sz.height;
 			unsigned int areaDiff = area - desiredArea;