[gl] Increase the color precision in TextureResize

My previous CL changed the texture data type used for resizing
to int instead of float. This caused noticable loss of color
precision in scaled textures.

Current CL deals with SwiftShader's refusal to draw into
GL_FLOAT texture in a different way - we use GL_HALF_FLOAT_OES
extension which is supported even in GLESv2; GL_HALF_FLOAT is
GLESv3-only.

Change-Id: I92688118393042bf106efa02b66a69967bf34530
2 files changed