Fix QPainterPath build issues

[echarlie]

readme fix, plus fix parallel to thestr4ng3r/chiaki#253 (based on quick googling of error) to get build on Arch Linux, qbs 1.16, qt 5.15.0, gcc 10.1.0

error from tests:

/home/eric/programs/cavewhere/testcases/test_CavewhereMainWindow.cpp:98: FAILED:
  CHECK( mainWindow2->property("x").toInt() == 323 )
with expansion:
  2974 (0xb9e) == 323 (0x143)

and a lot of noise about QML, e.g. file:///home/eric/programs/cavewhere/src/../qml/SaveFeedbackHelpBox.qml:22:5: QML Connections: Implicitly defined onFoo properties in Connections are deprecated. Use this syntax instead: function onFoo(<arguments>) { ... } and file:///home/eric/programs/cavewhere/qml/MainSideBar.qml:160: TypeError: Cannot read property 'left' of null

and reported leaks:

==3408721==ERROR: LeakSanitizer: detected memory leaks

Direct leak of 864 byte(s) in 6 object(s) allocated from:
    #0 0x7f9824d2ef41 in operator new(unsigned long) /build/gcc/src/gcc/libsanitizer/asan/asan_new_delete.cpp:99
    #1 0x7f98218d1204 in QQmlData::createQQmlData(QObjectPrivate*) (/usr/lib/libQt5Qml.so.5+0x25e204)

Indirect leak of 816 byte(s) in 6 object(s) allocated from:
    #0 0x7f9824d2f0c1 in operator new[](unsigned long) /build/gcc/src/gcc/libsanitizer/asan/asan_new_delete.cpp:102
    #1 0x7f9820914f04 in QHashData::rehash(int) (/usr/lib/libQt5Core.so.5+0x109f04)

Indirect leak of 288 byte(s) in 6 object(s) allocated from:
    #0 0x7f9824d2ef41 in operator new(unsigned long) /build/gcc/src/gcc/libsanitizer/asan/asan_new_delete.cpp:99
    #1 0x7f9820914d47 in QHashData::detach_helper(void (*)(QHashData::Node*, void*), void (*)(QHashData::Node*), int, int) (/usr/lib/libQt5Core.so.5+0x109d47)

Indirect leak of 192 byte(s) in 6 object(s) allocated from:
    #0 0x7f9824d2d459 in __interceptor_malloc /build/gcc/src/gcc/libsanitizer/asan/asan_malloc_linux.cpp:145
    #1 0x7f9820914b38 in QHashData::allocateNode(int) (/usr/lib/libQt5Core.so.5+0x109b38)

Indirect leak of 48 byte(s) in 6 object(s) allocated from:
    #0 0x7f9824d2ef41 in operator new(unsigned long) /build/gcc/src/gcc/libsanitizer/asan/asan_new_delete.cpp:99
    #1 0x7f98218d0e4e in QQmlData::attachedProperties() const (/usr/lib/libQt5Qml.so.5+0x25de4e)

SUMMARY: AddressSanitizer: 2208 byte(s) leaked in 30 allocation(s).