Resampler: For LPF orders to be a multiple of 2.

This is in preparation for an implementation simplification for the
purpose of some upcoming optimizations.

miniaudio.h

@@ -3017,7 +3017,9 @@ The linear resampler performs low-pass filtering before or after downsampling or
 depending on the sample rates you're converting between. When decreasing the sample rate, the
 low-pass filter will be applied before downsampling. When increasing the rate it will be performed
 after upsampling. By default a fourth order low-pass filter will be applied. This can be configured
-via the `lpfOrder` configuration variable. Setting this to 0 will disable filtering.
+via the `lpfOrder` configuration variable. Setting this to 0 will disable filtering. It should be
+set to a multiple of 2, such as 2, 4, 6, 8, etc. There are diminishing returns the higher you go.
+The maximum is `MA_MAX_FILTER_ORDER` which is 8 by default (it can be configured at compile time).
 
 The low-pass filter has a cutoff frequency which defaults to half the sample rate of the lowest of
 the input and output sample rates (Nyquist Frequency).
@@ -5580,7 +5582,7 @@ typedef struct
     ma_uint32 channels;
     ma_uint32 sampleRateIn;
     ma_uint32 sampleRateOut;
-    ma_uint32 lpfOrder;         /* The low-pass filter order. Setting this to 0 will disable low-pass filtering. */
+    ma_uint32 lpfOrder;         /* The low-pass filter order. Setting this to 0 will disable low-pass filtering. Must be a multiple of 2. */
     float     lpfNyquistFactor; /* 0..1. Defaults to 1. 1 = Half the sampling frequency (Nyquist Frequency), 0.5 = Quarter the sampling frequency (half Nyquest Frequency), etc. */
 } ma_linear_resampler_config;
 
@@ -5667,7 +5669,7 @@ struct ma_resampler_config
     void* pBackendUserData;
     struct
     {
-        ma_uint32 lpfOrder;
+        ma_uint32 lpfOrder; /* Must be a multiple of 2. */
     } linear;
 };
 
@@ -8718,7 +8720,7 @@ then be set directly on the structure. Below are the members of the `ma_device_c
     resampling.linear.lpfOrder
         The linear resampler applies a low-pass filter as part of its processing for anti-aliasing. This setting controls the order of the filter. The higher
         the value, the better the quality, in general. Setting this to 0 will disable low-pass filtering altogether. The maximum value is
-        `MA_MAX_FILTER_ORDER`. The default value is `min(4, MA_MAX_FILTER_ORDER)`.
+        `MA_MAX_FILTER_ORDER`. The default value is `min(4, MA_MAX_FILTER_ORDER)`. It must be a multiple of 2.
 
     playback.pDeviceID
         A pointer to a `ma_device_id` structure containing the ID of the playback device to initialize. Setting this NULL (default) will use the system's
@@ -58986,6 +58988,8 @@ static ma_result ma_linear_resampler_set_rate_internal(ma_linear_resampler* pRes
 
 static ma_result ma_linear_resampler_get_heap_layout(const ma_linear_resampler_config* pConfig, ma_linear_resampler_heap_layout* pHeapLayout)
 {
+    ma_uint32 lpfOrder;
+
     MA_ASSERT(pHeapLayout != NULL);
 
     MA_ZERO_OBJECT(pHeapLayout);
@@ -59002,6 +59006,11 @@ static ma_result ma_linear_resampler_get_heap_layout(const ma_linear_resampler_c
         return MA_INVALID_ARGS;
     }
 
+    lpfOrder = pConfig->lpfOrder;
+    if ((lpfOrder & 0x1) != 0) {
+        lpfOrder += 1;  /* Round up to even. */
+    }
+
     pHeapLayout->sizeInBytes = 0;
 
     /* x0 */
@@ -59025,7 +59034,7 @@ static ma_result ma_linear_resampler_get_heap_layout(const ma_linear_resampler_c
     {
         ma_result result;
         size_t lpfHeapSizeInBytes;
-        ma_lpf_config lpfConfig = ma_lpf_config_init(pConfig->format, pConfig->channels, 1, 1, pConfig->lpfOrder);  /* Sample rate and cutoff frequency do not matter. */
+        ma_lpf_config lpfConfig = ma_lpf_config_init(pConfig->format, pConfig->channels, 1, 1, lpfOrder);   /* Sample rate and cutoff frequency do not matter. */
 
         result = ma_lpf_get_heap_size(&lpfConfig, &lpfHeapSizeInBytes);
         if (result != MA_SUCCESS) {
@@ -59085,6 +59094,11 @@ MA_API ma_result ma_linear_resampler_init_preallocated(const ma_linear_resampler
     pResampler->lpfOrder         = pConfig->lpfOrder;
     pResampler->lpfNyquistFactor = pConfig->lpfNyquistFactor;
 
+    /* The order needs to be even. */
+    if ((pResampler->lpfOrder & 0x1) != 0) {
+        pResampler->lpfOrder += 1;  /* Round up to even. */
+    }
+
     pResampler->_pHeap = pHeap;
     MA_ZERO_MEMORY(pHeap, heapLayout.sizeInBytes);