Add render batch utility functions for depth control

Allows existing draw functions to be drawn out of order at different depths while still using batching.

* Add rlGetRenderBatchActive to match rlSetRenderBatchActive
* Add rlGetRenderBatchDepth to get the depth for the active render batch
* Add rlSetRenderBatchDepth to set the depth for the active render batch

src/rlgl.h

@@ -730,10 +730,14 @@ RLAPI int *rlGetShaderLocsDefault(void);                // Get default shader lo
 RLAPI rlRenderBatch rlLoadRenderBatch(int numBuffers, int bufferElements); // Load a render batch system
 RLAPI void rlUnloadRenderBatch(rlRenderBatch batch);    // Unload render batch system
 RLAPI void rlDrawRenderBatch(rlRenderBatch *batch);     // Draw render batch data (Update->Draw->Reset)
+RLAPI rlRenderBatch* rlGetRenderBatchActive(void);       // Get the active render batch
 RLAPI void rlSetRenderBatchActive(rlRenderBatch *batch); // Set the active render batch for rlgl (NULL for default internal)
 RLAPI void rlDrawRenderBatchActive(void);               // Update and draw internal render batch
 RLAPI bool rlCheckRenderBatchLimit(int vCount);         // Check internal buffer overflow for a given number of vertex
 
+RLAPI float rlGetRenderBatchDepth();                    // Get the depth for the active render batch
+RLAPI void rlSetRenderBatchDepth(float depth);          // Set the depth for the active render batch
+
 RLAPI void rlSetTexture(unsigned int id);               // Set current texture for render batch and check buffers limits
 
 //------------------------------------------------------------------------------------------------------------------------
@@ -3179,6 +3183,12 @@ void rlDrawRenderBatch(rlRenderBatch *batch)
 #endif
 }
 
+// Get the active render batch for rlgl
+rlRenderBatch* rlGetRenderBatchActive()
+{
+    return RLGL.currentBatch;
+}
+
 // Set the active render batch for rlgl
 void rlSetRenderBatchActive(rlRenderBatch *batch)
 {
@@ -3225,6 +3235,18 @@ bool rlCheckRenderBatchLimit(int vCount)
     return overflow;
 }
 
+// Get the depth for the active render batch
+float rlGetRenderBatchDepth()
+{
+    return RLGL.currentBatch->currentDepth;
+}
+
+// Set the depth for the active render batch
+void rlSetRenderBatchDepth(float depth)
+{
+    RLGL.currentBatch->currentDepth = depth;
+}
+
 // Textures data management
 //-----------------------------------------------------------------------------------------
 // Convert image data to OpenGL texture (returns OpenGL valid Id)