memory leaking due to camera movement

memory leaking due to camera movement

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Test plan:

Test it on a machine with nVidia card, go to a region with a lot of objects, stay static, move/fly around, zoom in/out, test the following three cases:
1, non-VBO: go to preferences -> graphics -> Hardware, disable VBO;
2, normal VBO: go to above link to enable VBO, go to "Debug settins" -> "RenderVBOMappingDisbaled", set it to be False;
3, VBO without mapping: repeat the case 2 setting, but set "RenderVBOMappingDisbaled" to be True.

before testing, go to "Advanced" -> "Show Info" -> "Show Memory" to display the memory usage; also press "ctrl+shift+1" to open the statistics console to watch FPS.

for each of the above three cases, test: 1, the memory usage, 2, the FPS value.

Expect: 1) the memory usage: case 1 uses least, and case 2 uses most, case 3 is in between. If "show info" -> "show render info" is set, the total vertex buffer size is displayed. the memory usage of case 3 minus the total vertex buffer size should be very close to that of case 1.
2, FPS: case 1 should be the slowest. case 2 is the fastest. and case 3 should be close to case 2.

Note: please let me know (bao@lindenlab.com) if the following happen:
1, the FPS of case 3 is consistently and noticeably slower than that of case 2;
2, crashes or drawing errors in VBO.
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