[rend2] Sort sky draws to prevent popping

When multiple skys are rendered per frame, they could pop randomly because of inconsitent sorting. Also makes sure sky stages are rendered after the actual sky image.

Author: SomaZ

shared/rd-rend2/tr_local.h

@@ -2479,6 +2479,7 @@ typedef struct {
 	backEndCounters_t	pc;
 	trRefEntity_t	*currentEntity;
 	qboolean	skyRenderedThisView;	// flag for drawing sun
+	byte		skyNumber;
 
 	qboolean	projection2D;	// if qtrue, drawstretchpic doesn't need to change modes
 	float		color2D[4];

shared/rd-rend2/tr_shade.cpp

@@ -2056,7 +2056,11 @@ static void RB_IterateStagesGeneric( shaderCommands_t *input, const VertexArrays
 
 		RB_FillDrawCommand(item.draw, GL_TRIANGLES, 1, input);
 
-		uint32_t key = RB_CreateSortKey(item, stage, input->shader->sort);
+		uint32_t key;
+		if (input->shader->sort == SS_ENVIRONMENT)
+			key = RB_CreateSortKey(item, stage+8, input->shader->sort);
+		else
+			key = RB_CreateSortKey(item, stage, input->shader->sort);
 
 		RB_AddDrawItem(backEndData->currentPass, key, item);
 

shared/rd-rend2/tr_sky.cpp

@@ -482,7 +482,8 @@ static void DrawSkySide( struct image_s *image, const int mins[2], const int max
 	RB_FillDrawCommand(item.draw, GL_TRIANGLES, 1, &tess);
 	item.draw.params.indexed.numIndices -= tess.firstIndex;
 
-	uint32_t key = RB_CreateSortKey(item, 0, SS_ENVIRONMENT);
+	int skyNumber = MAX(0, 7 - backEnd.skyNumber);
+	uint32_t key = RB_CreateSortKey(item, skyNumber, SS_ENVIRONMENT);
 	RB_AddDrawItem(backEndData->currentPass, key, item);
 
 	RB_CommitInternalBufferData();
@@ -867,6 +868,7 @@ void RB_StageIteratorSky( void ) {
 
 	// note that sky was drawn so we will draw a sun later
 	backEnd.skyRenderedThisView = qtrue;
+	backEnd.skyNumber++;
 }