A previous change didn't really ignore the return value of fread() and
a warning was still emitted:
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[103/1571] Building C object src/xrt/drivers/CMakeFiles/drv_ns.dir/north_star/ns_hmd.c.o
../src/xrt/drivers/north_star/ns_hmd.c: In function ‘ns_config_load’:
../src/xrt/drivers/north_star/ns_hmd.c:512:8: warning: ignoring return value of ‘fread’, declared with attribute warn_unused_result [-Wunused-result]
512 | (void)fread(json, 1, file_size, config_file);
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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Check the return value of fread to fix the warning for good, and declare
the return value of ftell as long as stated in the man page.
On some systems fread() is declared with attribute warn_unused_result
and this results in a build warning:
-----------------------------------------------------------------------
Building C object src/xrt/drivers/CMakeFiles/drv_ns.dir/north_star/ns_hmd.c.o
.../src/xrt/drivers/north_star/ns_hmd.c: In function ‘ns_config_load’:
.../src/xrt/drivers/north_star/ns_hmd.c:512:2: warning: ignoring return value of ‘fread’, declared with attribute warn_unused_result [-Wunused-result]
512 | fread(json, 1, file_size, config_file);
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-----------------------------------------------------------------------
Explicitly ignore the return value of that fread() to silence the
warning.
And while at it also close config_file in some error paths between
fopen() and fclose() which where leaking the file pointer when jumping
to the parse_error label.
This matches the OpenXR usage: the array is the plural of the element type,
and the count is the singular element type plus "count" (usually CountOutput
because of the two-call idiom)
Includes fixes to other code to match API changes.
Each HMD driver now has to implement compute_distortion() which will be called
by the compositor implementation to generate a mesh (usually).
u_distortion_mesh contains implementations for the defaults (panotools, OpenHMD, vive).
Also adds compute_distortion function for Vive distortion
There are differences between OpenHMD and Panotools values, main differences for now:
* psvr has 5 pano coefficients, ohmd has 3
* psvr uses viewport size and lens center in pixels for distortion calculation, ohmd in meter
* psvr uses different distortion scaling than ohmd
