make use of sample loop flag

issue #1109

this MAY BREAK THINGS
if any problems, REVERT THIS ONE

src/engine/platform/qsound.cpp

@@ -319,7 +319,7 @@ void DivPlatformQSound::tick(bool sysTick) {
       if (length > 65536 - 16) {
         length = 65536 - 16;
       }
-      if (loopStart == -1 || loopStart >= length) {
+      if (!s->isLoopable()) {
         if (i<16) {
           qsound_end = offPCM[chan[i].sample] + length + 15;
         } else {

src/engine/platform/segapcm.cpp

@@ -138,7 +138,7 @@ void DivPlatformSegaPCM::tick(bool sysTick) {
             rWrite(0x84+(i<<3),(sampleOffSegaPCM[chan[i].pcm.sample])&0xff);
             rWrite(0x85+(i<<3),(sampleOffSegaPCM[chan[i].pcm.sample]>>8)&0xff);
             rWrite(6+(i<<3),sampleEndSegaPCM[chan[i].pcm.sample]);
-            if (loopStart<0 || loopStart>=actualLength) {
+            if (!s->isLoopable()) {
               rWrite(0x86+(i<<3),2+((sampleOffSegaPCM[chan[i].pcm.sample]>>16)<<3));
             } else {
               int loopPos=(sampleOffSegaPCM[chan[i].pcm.sample]&0xffff)+loopStart;
@@ -156,7 +156,7 @@ void DivPlatformSegaPCM::tick(bool sysTick) {
             rWrite(0x84+(i<<3),(sampleOffSegaPCM[chan[i].pcm.sample])&0xff);
             rWrite(0x85+(i<<3),(sampleOffSegaPCM[chan[i].pcm.sample]>>8)&0xff);
             rWrite(6+(i<<3),sampleEndSegaPCM[chan[i].pcm.sample]);
-            if (loopStart<0 || loopStart>=actualLength) {
+            if (!s->isLoopable()) {
               rWrite(0x86+(i<<3),2+((sampleOffSegaPCM[chan[i].pcm.sample]>>16)<<3));
             } else {
               int loopPos=(sampleOffSegaPCM[chan[i].pcm.sample]&0xffff)+loopStart;

src/engine/platform/snes.cpp

@@ -222,7 +222,7 @@ void DivPlatformSNES::tick(bool sysTick) {
           if (chan[i].audPos>0) {
             start=start+MIN(chan[i].audPos,s->lengthBRR-1)/16*9;
           }
-          if (s->loopStart>=0) {
+          if (s->isLoopable()) {
             loop=((s->depth!=DIV_SAMPLE_DEPTH_BRR)?9:0)+start+((s->loopStart/16)*9);
           }
         } else {

src/engine/vgmOps.cpp

@@ -591,7 +591,7 @@ void DivEngine::performVGMWrite(SafeWriter* w, DivSystem sys, DivRegWrite& write
               w->writeC(0x95);
               w->writeC(streamID);
               w->writeS(write.val); // sample number
-              w->writeC((sample->getLoopStartPosition(DIV_SAMPLE_DEPTH_8BIT)==0)|(sampleDir[streamID]?0x10:0)); // flags
+              w->writeC((sample->getLoopStartPosition(DIV_SAMPLE_DEPTH_8BIT)==0 && sample->isLoopable())|(sampleDir[streamID]?0x10:0)); // flags
               if (sample->isLoopable() && !sampleDir[streamID]) {
                 loopTimer[streamID]=sample->length8;
                 loopSample[streamID]=write.val;
@@ -610,7 +610,7 @@ void DivEngine::performVGMWrite(SafeWriter* w, DivSystem sys, DivRegWrite& write
             w->writeC(0x95);
             w->writeC(streamID);
             w->writeS(pendingFreq[streamID]); // sample number
-            w->writeC((sample->getLoopStartPosition(DIV_SAMPLE_DEPTH_8BIT)==0)|(sampleDir[streamID]?0x10:0)); // flags
+            w->writeC((sample->getLoopStartPosition(DIV_SAMPLE_DEPTH_8BIT)==0 && sample->isLoopable())|(sampleDir[streamID]?0x10:0)); // flags
             if (sample->isLoopable() && !sampleDir[streamID]) {
               loopTimer[streamID]=sample->length8;
               loopSample[streamID]=pendingFreq[streamID];

src/gui/sampleEdit.cpp

@@ -219,12 +219,17 @@ void FurnaceGUI::drawSampleEdit() {
         if (ImGui::Checkbox("Loop",&doLoop)) { MARK_MODIFIED
           if (doLoop) {
             sample->loop=true;
-            sample->loopStart=0;
+            if (sample->loopStart<0) {
-            sample->loopEnd=sample->samples;
+              sample->loopStart=0;
+            }
+            if (sample->loopEnd<0) {
+              sample->loopEnd=sample->samples;
+            }
           } else {
             sample->loop=false;
+            /*
             sample->loopStart=-1;
-            sample->loopEnd=sample->samples;
+            sample->loopEnd=sample->samples;*/
           }
           updateSampleTex=true;
           if (e->getSampleFormatMask()&(1U<<DIV_SAMPLE_DEPTH_BRR)) {